What is the role of a global catalog in Active Directory?

Answer 1

"The global catalog is a distributed data repository that contains a searchable, partial representation of every object in every domain in a multidomain Active Directory forest. The global catalog is stored on domain controllers that have been designated as global catalog servers and is distributed through multimaster replication. Searches that are directed to the global catalog are faster because they do not involve referrals to different domain controllers." http://technet.microsoft.com/en-us/library/cc728188(WS.10).aspx

Answer 2

The global catalog is a domain controller that contains attributes for every object in the Active Directory. By default, only the members of the Schema Admins group have rights to change which attributes stored in the global catalog, according to organization's requirements. The global catalog contains: The commonly used attributes need in queries, such as a user's first and last name, and logon name.

All the information or records which are important to determine the location of any object in the directory.

A default subset of attributes for each object type.

All the access related permissions for every object and attribute that is stored in the global catalog. Say, without permission you can't access or view the objects. If you are searching for an object where you do not have the appropriate permissions to view, the object will not appear in the search results. These access permissions ensure that users can find only objects to which they have been assigned access. A global catalog server is a domain controller that contains full and writable replica of its domain directory, and a partial, read-only replica of all other domain directory partitions in the forest.