As a land with many nobles, Cormyr has a number of titles and considerations thereof to consider. The Dalelands also has their leadership, but this is to a much lesser extent.
This list is not exhaustive
Royal Family
Obarskyr
Royal Houses (Noble houses related by blood to the Royal Family)
Crownsilver, Huntsilver, Truesilver
Others
Blacksilver, Dauntinghorn, Huntcrown, Bracegauntlet, Blackwagon, Belstable, Calantar, Dracohorn, Ebonhawk, Emmarask, Goldwyn, Greenmantle, Hardcastle, Hawklin, Hornshield, Ireabor, Jacyllyth, Jarnuxlin, Lionsyn, Longbrooke, Marliir, Nelduk, Rallyhorn, Roaringhorn, Rowanmantle, Santedul, Seshore, Silverhorn, Skatterhawk, Tapstorn, Thundersword, Tilthar, Wyvernspur
**
Past Houses banished and stripped of lands/titles**
Cormaeril, Bleth. Illance, Silversword, Goldsword, Tathcrown
First off, it is important to note that there are several degrees and types of nobility in Cormyr.
Hereditary Peers - these are often what one thinks of when referring to "The Nobility." These are Cormyr's hereditary noble houses, who own large estates throughout the kingdom. Families such as Wyvernspur, Truesilver, Huntcrown, are among the most prominent of these, though there are others, including several whose estates are near Arabel, and maintain residency there.
These are universally pureblooded humans, due to the predilection of the aristocracy for carefully chosen marriages within "acceptable" circles. New Peers may only be created by a land grant from the Crown, an exceedingly unusual act.
The King's Lords - These are appointed officials who serve as rulers in cities and towns (aside from those not governed by separate Crown charter, such as Arabel) in the King's name, and whose status is delegated from the Crown directly.
Although they possess the same priviledges and are due the same courtesies as the noble peers, the King's Lords do not pass this status to their children, nor are they considered to be head or part of a House. (Tessaril Winter is the King's Lord of Eveningstar, and is referred to as Lord Winter, but her children will not automatically be granted this status, nor is there a "House Winter".)
Baronetcy - These are non-hereditary grants of lesser noble status by the Crown to those who have performed exceptional service to the kingdom, titled as "Baronet" or Baronetess". Socially they are slightly above Knights, use a "Lord/Lady" form of address, and are expected to provide assistance to the Crown when called on; however unlike Knights, that is not expected to be martial in nature.
Cormyrean Knights - Knighthood is a status that can be granted by the Crown and certain Noble Houses for exceptional service or deeds. There are many varieties of this, though all share certain characteristics:
Note that this can be confusing, as many religious orders have "Knights" that are not considered part of the Noble Class. In general, referring to someone as a "Knight" can cover all of these, or just refer to mounted warriors in general, whereas "Knight of Cormyr" or "Knight of House (Wyvernspur/etc)" refer to specific noble knights. Note too that Knights need not necessarily be armored horsemen, etc.
A few specific examples, within Cormyr:
Cormyrean Highknights - Highknight is a title given to Cormyrean knights who have shown exceptional service to the Crown. Many highknights gravitate towards court assignments, while others remain on the battlefield, serving alongside their fellow Purple Dragons.
Purple Dragon Knight - Also called Knights of Cormyr, those knighted by the Crown are also considered to be part of the Royal Army, as part of their feudal duties, and may either serve as part of regular units, or as Knights Errant serving special purposes. They may also possess a military rank as an Officer (Company or Field Grade), though not all Officers are Knights. Senior Officers (rank of Constal and above) tend to be Knights. Note that "Purple Dragons" refer to the Army as a whole, common soldiers, officers, and knights included, while "Purple Dragon Knights" are Knighted individuals who also belong to the army.
Titles used in other realms such as Duke, Baron, etc are largely honorific in Cormyr, as nobles do not rule their own fiefdoms or otherwise possess regions of land. Generally these titles are non-hereditary, though they are often given to members of hereditary noble houses.
Inheritance Laws
A hereditary noble’s title is affected most strongly by the circumstances of that noble’s birth: the rank of the parent holding the relevant title, the order of birth, and the family’s position with the Crown. Titles are traditionally inherited by absolute cognatic primogeniture, with the eldest child inheriting, passing first to their direct heirs in birth order if deceased, and then on to the second born child followed by their offspring, and so forth.
Certain circumstances can remove someone from this order of inheritance, such as renouncing inheritance to take religious vows, or by marriage to the heir of another house (which would be decided as part of the marriage contract which of the two did so). Although this is custom, Cormyrian law ultimately allows the final decision of who inherits to fall to the title holder, even if bypassing the birth order risks severe scandal. In such cases, potential heirs may be deliberately passed over, disinherited outright, or even disowned entirely.
On Fealty
Nobles cannot have their own subject vassals. Instead, all nobles are direct vassals to the Crown. Though they may have guards and such in their employ, such a thing is strictly limited.
Loss of Title
In extreme circumstances such as treason or rebellion, the monarch may strip the titles from a particular head of the house, either to pass to a worthy heir (direct bloodline or of the monarch's choosing), or remove the offending House entirely (as happened to the Houses of Cormaeril and Bleth).
Although Cormyr is not a true feudal state where nobles possess and rule fiefdoms of their own, it nonetheless has a strong and storied noble class which, while lacking direct authority, comprise the upper echelons of Cormyrian society and on whom the Crown does rely primarily for its appointed officers and officials. The etiquette related to forms of address and titles is thus slightly different than a typical medieval setting.
Methods of Address:
Nobles above the rank of Knight may also be referred to as Lord or Lady, though this tends to be less formal or used in third person reference. Note too that "The Lord/Lady Crownsilver" would explicitly refer to the head of the house, and would not apply to other members of the family.
Knights are referred to as Sir or Dame, and often use this form of address even if possessed of a higher status, e.g. Ilberd Crownsilver is usually referred to as Sir Ilberd Crownsilver rather than Lord, to reflect that the knightly title is his personal right rather rather than something derived from his status as a member of the house.
Third person address follows similarly, though one might combine both if applicable (e.g. for Kimba Crownsilver one might use "Her Grace Lady Crownsilver", "Her Grace", or simply "(the) Lady Crownsilver", but only "Lady Crownsilver" would apply for her granddaughter).
In terms of formal titles (such as used for announcing them at Court, or in legal documents, formal letters, et cetera), only the head of House is titled as Lord/Lady, unless the individual has earned a separate and personal honorific.
Example: Ayesunder Truesilver, head of House Truesilver, is "His Grace Ayesunder (various middle names), Lord of House Truesilver, Warden of the Port of Marsember, Admiral of the Blue Dragons", but his daughter and direct heir Glorasta is simply "Glorasta (various middle names) of House Truesilver". Comparatively, Ilberd Crownsilver is "Sir Ilbred (various middle names) of House Crownsilver, Knight of the Realm".
All Cormyrian nobles, from Knights on up, are entitled to their own personal coat of arms. Note that noble PCs are assumed to have such, and newly knighted or ennobled PCs would be entitled/expected to establish their own with the Heralds of the kingdom (who would ensure that the chosen symbols do not conflict with any existing such).
In practical/OOC terms, Players of such characters may do so if they wish but are not required to (it will simply be assumed that they do so offscreen if they don't want to). That is, designing/describing a coat of arms is completely optional for players, as a fun thing, and is in no way required.
One of the key important considerations to any noble family is that of inheritance. Marriage and inheritance are how noble families seal alliances and seek to gain status. Because of this, marriage and children take on a special importance far exceeding that which it does for common folk. Noble children are expected to marry for duty and family, not for love, although that can be a consideration provided that they chose a suitable match. Noble families keep meticulous records of births, marriages, and so forth, as who is related to who matters quite a bit.
That said, nobles (or at least human nobles) are often prone to dalliances and affairs outside of marriage, in part due to the fact that many noble marriages are made for considerations other than love. This is considered socially acceptable IF it is kept quiet enough (most everyone does it, so as long as it doesn't become public spectacle and scandal, it's not a big deal). In most cases, the ready supply of herbs that can be used to dampen fertility tends to limit the number of children born to such trysts; however, such does occur, particularly if such precautions are not taken. In the case of children born in such a manner, there are several possibilities:
Additionally, in cases of illicit trysts among nobles, such children if unacknowledged may well grow up as part of a different house, unaware of their true father (such was the case for several of Azoun IV's bastards). In this case they tend to be treated as the child of their public parents in all respects (though rumors may swirl in private). In the case that such children are acknowledged, this can become rather complicated, although depending on the circumstances they may still be treated as part of the birth house.
Because of the importance of considerations of inheritance, bloodlines, and the matters of alliances thereof, noble families almost never adopt. If they do, it is usually in the sense of a childless head of the house adopting a blood-related child from another branch of the family as a way of designating them as heir. In the case of non-blood relatives, noble families will almost always take in such children as "Wards" of the house, who are treated in a manner similar to Acknowledged Bastards, but without the possibility of later being declared legitimate. Additionally, legitimized offspring may sometimes be declared "adopted" by the non-blood related parent as a formality.
Generally speaking, most nobles are extremely particular about matters of bloodline and pedigree and would be aghast at the thought of a noble marrying a commoner. The thought of marrying outside their race/species is even more scandalous in the extreme, and the more extreme the difference the greater the negative reaction. While the degree of reaction may vary quite a bit among particular nobles, as well as with which race/country. For instance, many Cormyrian nobles would turn their nose up at a noble who married an elf, or at the half-elven child of such a union, and might gossip about such, but by comparison, most Drow houses wouldn't consider a Half-Drow to even BE nobility at all, beyond the degree they might be forced to by the power of the House in question. This tends to fade as the separation increases, so the child of a human and half-elf would face far less prejudice and negativity, limited largely to a few traditionalists (at as long as they then married another human noble rather than someone elf-blooded). Conversely, more extreme examples will be reacted to much more negatively, with an example of the opposite extreme being the Fey'ri, who were actively working to mix demonic blood into various elven house bloodlines and were met with violent opposition from other elves.
As such, racially exogamous marriages among nobility are all but unheard of, and noble children not of the appropriate race are almost exclusively illegitimate in some way, at least to begin with.
As Faerûn tends to be relatively open-minded on such things, marriages that are not expected to produce offspring (such as same-sex unions) are generally accepted even if not considered necessarily preferrable from the standpoint of nobility. Such unions can provide alliances and such even if not providing children, but especially in houses with many alternate heirs, this is not necessarily seen as a bad thing. Such cases tend to be somewhat of an exception to the above considerations on racially exogamous unions (since no one is worried about what it means for the bloodlines). In the case of nobles expected to inherit, it is common to adopt a relative's child as their heir, or for an illegitimate child of the noble heir to be legitimized. It is widely frowned upon among the nobility to employ magic to conceive offspring (this is the case regardless of the parents), though that isn't to say it doesn't happen - just that those who do tend to publicly treat the child as being a 'normal' out of wedlock child that then gets legitimized, and formally adopted by the other parent.
It is fairly rare to see PC Nobles on CD. This has mainly to do with the advanced social status and authority it would provide, although there are exceptions, mainly when the status is somehow compromised, weakened, or kept in check.
So how do I get to be one?
Note that Noble status is conditional on not abusing it - characters that attempt to throw their status around, use it against other PCs, and so on, are liable to have their status revoked in some way or other.
In-character authority over other characters is something that is only bestowed with direct approval from the staff. This is primarily why noble status and similar things require applications, and promotions within groups such as the Purple Dragons are similarly only given out by staff. If you want to hold a position or title, it needs to be approved.
Titles & Authority
Note that minor/honorary titles that are bereft of any authority are usually fine. For instance, a Cleric PC that decides to go by the title "Father" or "Elder" or "Sister" (or the like) isn't a big deal. Declaring yourself to be an Archbishop and either implying or outright stating that this gives you religious authority over other PCs, however, would NOT be okay. Playing a Knight of a Religious Order (with no authority or status other than "hey I'm a warrior for the church") is generally fine, but being a Knight of the Realm (and thus minor nobility) requires approval.
Additionally, character level is never considered equivalent to any IC rank, whether in comparison to NPCs, or to other PCs. Expect that NPCs especially may be of lower level but of greater authority.
When it comes to player-created groups (Adventuring companies, troupes, secret cults, etc), you're free to make up whatever titles you want for yourselves, so long as you remember that they don't grant any special status or authority outside the group. That is, if PCs agree to work for you and call you boss, then that sort of thing is fine because they're choosing to voluntarily follow you, and it isn't being imposed on anyone, etc.
Resources
Furthermore, characters are not allowed to command resources beyond themselves. That is, you can't just declare that you're calling on a thousand templar knights of the Triad, even if you have been approved for a position of some reverence or authority. Similarly, players from a noble or wealthy background cannot directly command resources (be they monetary or other) simply because they're related to House Soandso, or because they're from some merchant family in Amn, you have to go to the NPCs that do (i.e. your parents/grandparents/uncles/aunts/etc, which are an absolute requirement for a family/etc possessing such resources in the first place).
You may attempt to ask NPCs for appropriate assistance in various circumstances, but the bigger or more outlandish the request the more likely it is to be denied. That is, you're far more likely to have the Triadic Church agree to pass along your pleas for help than you are to get them to send a crusading army, and you're far more likely to get your rich merchant uncle to introduce you to some of his contacts in a particular city than you are to get him to spend a bunch of money buying food for refugees. Note too that these NPCs are persuadable (at the discretion of the DM), and your rich merchant uncle/parents might well agree to make an investment you recommend if they think there's significant profit potential there. Even then, too, there are limits to what resources will be available no matter how persuasive or how enticing an opportunity, because no one has infinite wealth/manpower/etc.
In any case such responses are 100% up to the DM in a particular quest, and if they refuse, that's it - your character will have to stand on their own.