Global Catalog

A domain controller that contains a partial replica of every domain in Active Directory. In other words, a global catalog holds a replica of every object in Active Directory, but with a limited number of each object's attributes. The global catalog stores those attributes most frequently used in search operations (such as a user's first and last names) and those attributes required to locate a full replica of the object.
The Active Directory replication system builds the global catalog automatically. The attributes replicated into the global catalog include a base set defined by Microsoft. Administrators can specify additional properties to meet the needs of their installation.