What is a global catalog (GC)?

The Microsoft Windows 2003 Active Directory glossary defines a global catalog (GC) as “the global catalog contains a partial replica of every Windows 2003 domain in the directory. The GC lets users and applications find objects in an Active Directory domain tree given one or more attributes of the target object. It also contains the schema and configuration of directory partitions. This means the global catalog holds a replica of every object in the Active Directory, but with only a small number of their attributes. The attributes in the global catalog are those most frequently used in search operations (such as a user’s first and last names, logon names, and so on), and those required to locate a full replica of the object. The GC allows users to find objects of interest quickly without knowing what domain holds them and without requiring a contiguous extended namespace in the enterprise. The global catalog is built automatically by the Active Directory replication system.”