A character that is not primarily and actively played by a player is called an NPC. Even though they are not the primary character of a player, they can be quite important to the story and to the game.
In some games, the Game Master (GM) does not play a character of his own in the game. Then he can use a number of NPCs to introduce problems or to move the story in a certain direction. It can for example be the wealthy merchant that hires the players to protect a shipment of gold, or it can be the shady rogue the players hire to guide them through the forest. In any way, the Game Master can use the NPC to have the players move into the intended direction of the adventure.
Depending on the rules of the game, players may be allowed to create NPCs themselves. If for example, for some reason their regular character is not suitable for a specific idea. It may also be a good idea to create a NPC to post for to get a change from posting for your regular character, a change of scenery of sorts. The specific rules concerning NPCs varies with the game, so be sure to consult your Game Master on this.
From my experience, NPCs can be categorized in two ways: by their importance and by who ‘owns’ it. An important NPC is often given a proper name, a personality and a more detailed description while an NPC that the players will not meet again does not receive the same attention.
Named NPCs – An NPC that is important to the story, a key character, will appear in several posts and perhaps over a longer period of time – maybe even spanning over several adventures. Such an NPC is most often given a name (but not always) or at least a title to identify that it indeed is the same character.
Unnamed NPCs – A character that will most likely never appear again, or does not play important enough role to warrant a name, is preferably left unnamed. While naming an NPC adds to the ‘feeling’ of the person and story, it can be problematic in the long run. Usually it is enough to refer to them for example by characteristics (“the brown-haired guy”) or by its function or title (“The barkeeper”). Giving them names can ‘tag’ them as NPCs that can (and perhaps will) return. Having too many named characters “on standby” can be confusing and hard to keep track of.
The second way to categorize NPCs is by who owns and/or created it. The rules for this are usually different from game to game, but in general an NPC can either be either restricted (“owned”) or free-for-all.
An NPC that is considered restricted is usually owned by someone, either a player or the Game Master. They decide what happens to it. Depending on the game rules, these characters can almost be seen as a player’s secondary characters. NPCs controlled by the Game Master can usually be considered ‘untouchable’.
An NPC character that is “free-for-all” can be used by all players in their posts. They are usually NPCs that are very general and plays only a supporting role in the game. It does not matter to the story in general if something would happen to the character.
Unless a character is specifically said to be free-for-all, it may be a good idea to ask the Game Master for permission to use a specific NPC if what you intend to post can have any long term effect on it. It is often safe to apply the same rules as you would to using another player’s character. Take care when considering what to write and as for permission if you are unsure.