An abbey is the architectural complex of a religious community. An abbey is known specifically as a monastery (if inhabited by monks) or a convent (if inhabited by nuns), and is led respectively by an abbot or abbess.
The notion of living a secluded Christian life of spiritual contemplation originated in the far eastern Mediterranean during the first centuries AD. Here and there, a devoted follower would move to the wilderness, becoming a solitary hermit dwelling in a simple house or cave. Others, in search of a similarly meditative life but less inclined to such extreme isolation, joined up to form religious communities.
The home of a religious community, known as an abbey, is necessarily self-supporting, and thus tends to resemble a small village. The standard buildings of an abbey include the church, dormitories, kitchen, refectory (dining hall), chapter house (meeting room), hospital, workshops, and storage rooms.1
Another standard feature is a central courtyard, secured from intrusion simply by being surrounded by the community's buildings (and, in some cases, defensive walls). The courtyard of an abbey is often a cloister: a courtyard framed by covered walkways. Larger abbeys might feature multiple courtyards.1
The first abbey in Western Europe was Montecassino Abbey, established in the Dark Ages by Saint Benedict of Italy. Saint Benedict founded Western Europe's first major monastic order, the Benedictine Order, whose organization and lifestyle became the general template for subsequent religious orders throughout Western Europe. Though Montecassino (which lies south of Rome) exhibits the standard European abbey layout, its buildings are not original, having been repeatedly destroyed and rebuilt throughout the centuries.1,3
Note that sometimes a lone church is referred to as an "abbey" (e.g. Westminster Abbey). This indicates that the building, once an abbey church, is the only part of the abbey to have survived.1
2 - "Western Architecture: The Christian West » Romanesque", Encyclopedia Britannica. Accessed May 2009.
3 - "Monte Cassino", Encarta. Accessed May 2009.