A group or faction of players or NPCs, usually in role-playing games (RPGs), where members look after one another and embark on quests for or with each other.
The term has very different applications in single-player gaming compared with online gaming. In single-player games, a guild is simply a social group of NPCs within the game world that can be joined or worked for to gain special rewards (e.g. Morrowind). In online games such as MMORPGs, a guild is usually organised and run completely by other players (eg. World of Warcraft).