Global Catalog

A directory database that applications and clients can query to locate any object in a forest. The global catalog is hosted on one or more domain controllers in the forest. It contains a partial replica of every domain directory partition in the forest. These partial replicas include replicas of every object in the forest, as follows: the attributes most frequently used in search operations and the attributes required to locate a full replica of the object. [Exchange 2003]

Allows forests to share a portion of their entire AD database. GC supports research and indexing of forest-wide information.