An Active Directory site is a physical subnet that is connected using a reliable, high-bandwidth connection. An Active Directory site structure represents the physical structure of your network and is separate from the logical structure of the network, which is represented by forests, domains, and organizational units. Sites are used to designate replication boundaries and isolate logon authentication traffic between physical network locations.
A single Active Directory site can contain resources from different Active Directory domains, and a single Active Directory domain can exist across different Active Directory sites.
You should create additional Active Directory sites to control Active Directory replication traffic and to isolate logon traffic.
Remember, an Active Directory site is connected using reliable, high-bandwidth connection. Each site should have at least one Active Directory Domain Controller and one Global Catalog, to avoid using low bandwidth WAN connection for Active Directory replication traffic and to isolate logon traffic.
Each Active Directory site should have at least one DNS server and one DHCP server for name resolution and to assign automatic IP setting to computers.