The Global Catalog contains a partial replica of the domain Active Directory for every domain in an enterprise forest. The Global Catalog server replicates a copy of all objects from every domain in the forest, but only contains a subset of each object's attributes. The attributes that are replicated by default are those that are those used in most common queries. The Global Catalog provides one information store that can be queried to locate forest-wide information.
You can change the Global Catalog contents by using the Schema Management snap-in on a schema-writeable domain controller. Select the attribute that you want to add to the Global Catalog and open the properties for the object. There is a check box named "Replicate this attribute to the Global Catalog" that you can select or clear to add or remove objects.
Note that the number of objects stored in the Global Catalog affects the replication time of domain controllers that are Global Catalog servers. Exercise caution in adding attributes to your Global Catalog because this can add attributes for the object from every domain in the forest to the Global Catalog.