Microsoft® Windows® 2000 Scripting Guide
Promoting a computer running Windows 2000 Server to a domain controller installs Active Directory on the computer. The Active Directory store is contained on each domain controller in the domain. Before the promotion process completes, you must specify the relationship of the new domain controller to the network. The domain controller can be configured to create a domain or to become part of an existing domain.
To make Active Directory scalable from a single-server network to a network containing thousands of servers, Active Directory is designed to contain many domains. All domains are interconnected to form a hierarchy. A single branch in the hierarchy is called a tree, and all the trees in the hierarchy are collectively called a forest.