Basic Information
A meta concept, mainly intended for fantasy RPGs (although it can be brought into the modern era by employing the PCs as private security contractors for an Executive Outcomes or Blackwater analogue). The PCs are members of a guild of fighters which assigns them a variety of combat based missions for assorted clients.
This allows a good variety of work to be assigned, and can make adventure hooking a lot easier as there is no need for the PCs to be arbitrarily gathered in a tavern.
The GM may or may not create a campaign arc in which some or all of the missions turn out to be related and/or mini arcs in which a client contracts for a series of jobs with a given theme.
The downside being that - particularly in class/level obsessed systems - it can be hard to assemble an adventuring-party as some skills will be rare or impossible to find in a given fighters guild. In a low fantasy or historical campaign - in which most units don't expect to have a fighter, a wizard, a thief and a cleric and there are no hard-coded laws stopping you from learning skills not directly related to your primary job - this is less of a problem.
Similar guild based campaigns can be arranged for wizards, thieves etc. or the GM can simply institute an "adventurer's guild" and leave it at that.