A site is a combination of one or more Internet Protocol (IP) subnets that are connected by a high-speed link. Defining sites allows you to configure the Active Directory access and replication topology so that Windows 2000 uses the most efficient links and schedules for replication and logon traffic.
You create sites for two primary reasons:
Sites map the physical structure of your network, whereas domains map the logical structure of your organization. The logical and physical structures of Active Directory are independent of each other, which has the following consequences: