Troupe Play refers to a style of group roleplaying that was most strongly developed in Ars Magica but strongly influences all of the systems that have developed from it (including most Storytelling Games). Troupe Play is generally defined by a group of players (the Troupe) who have common resources, both as characters and players.
In Ars Magica, the Troupe unit was the Covenant, the mystic organization that the mages developed. Each covenant has a physical location and lifecycle (represented seasonally in AM), characters joined and left the covenant, shared common resources, and played out their games with the covenant serving as a home base. Most White Wolf games have some concept of a troupe community, either loosely defined (such as the Coteries in VtM) or well-supported in the rules (such as wu in Kindred of the East).