A Non-Player Character, more commonly known as an NPC, is a character in a computer roleplaying game that is not controlled by a Human player. Typically, NPCs are only neutral or allied characters and not enemies of players.
The nature of an NPC varies by game type. In MMORPGs, NPCs are usually lack any form of scripting and exist only for decoration, as Vendors or as Quest-givers. They typically lack any form of interactive dialogue and will typically either only play audio clips or bring up the appropiate Quest/Vending screen. In Play By E-mail and Play by Forum RPGs, the term NPC refers to a character that has at some point been created and brought into play by a Player. These characters are usually under the control of one or more players. Finally, in Video Game RPGs an NPC can refer to any non-hostile character (and sometimes hostile characters) that is a member of a non-beast/creature species.
Non-hostile NPCs are typically invicible unless able to engage in combat with certain players, such as those of an opposing faction. The exception is in Forum/E-Mail RPGs, where unimportant NPCs can be killed at the whim of the player. NPCs of more importance can usually be killed with the creator's direct permission.