The Active Directory Global Catalog is the central storage of information about objects in an Active Directory forest. A Global Catalog is created automatically on the first domain controller in the first domain in the forest. The Domain Controller which is hosting the Global Catalog is known as a Global Catalog Server. A Global Catalog server stores a full copy of all objects in the directory for its host domain and a partial copy of all objects for all other domains in the forest. Global Catalog helps in searching Active Directory objects in the foreset more efficiently.
The Active Directory Global Catalog is responsible for several other important functions of the Active Directory, such as the following:
• Logon validation of universal group membership
• User Principal Name (UPN) logon validation through DC location
• Search capabilities for every object within an entire forest
The function of a Global Catalog can be compared with a telephone directory. Global Catalog stores information like a telephone directory that users can perform queries against to find specific information.