The Active Directory catalog is simply a distributed listing of all objects within the enterprise, along with the attributes assigned to each. An object might be a user, group, printer, or computer. Attributes are defined qualities for each, such as a user's name and login, or a printer's location and asset identification. The schema defines the available attributes for objects listed within the catalog as each is created.
Schema changes should only be made after a thorough review by senior administrators. Typically, schema changes are only enacted by software installations such as the domain-prep and forest-prep procedures required for Exchange 2000 installations.
Searching for resources is made much simpler by this process. As an example, an administrator could search the global catalog looking for users with a last name of Jones in order to find a particular usera much easier solution than searching through the user listings for each domain linked via trust in older versions of Windows.