A domain controller stores domain information related to its domain alone. To locate objects outside its domain is beyond its scope. Hence there is a need for a Global Catalog server. The global catalog contains a partial representation of all objects in the entire forest. Hence it has the potential to search objects from any domain.
The global catalog is built and updated automatically by the AD replication system. Only certain attributes of the objects which are likely to be searched in the queries are updated to these servers. This helps in speeding up the entire process. The global catalog is stored on domain controllers that have been designated as global catalog servers.