Microsoft Definition of the Global Catalog
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 Domain Services (AD DS) 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.
Please see the following URLs for more information on Microsoft Global Catalogs
What is the Global Catalog?
Understanding the Global Catalog
How the Global Catalog Works
Related documents from Microsoft
How do I check / enable the Global Catalog on a DC?
Determine Whether a Domain Controller Is a Global Catalog Server