Domains are units of replication. In addition, all of the domain controllers in a particular domain can receive changes and replicate those changes to all other domain controllers in the domain. Each domain in Active Directory is identified by a (DNS) Domain Name System domain name and requires one or more domain controllers. If your network requires more than one domain, you can easily create multiple domains.
One or more domains that share a common schema and global catalog are referred to as a forest.
The first domain in a forest is referred to as the forest root domain. If multiple domains in the forest have contiguous DNS domain names, then the structure is referred to as a domain tree.
A single domain can span multiple physical locations or sites and can contain millions of objects. Site structure and domain structure are separate and flexible. Furthermore, a single domain can span multiple geographical sites, and a single site can include users and computers belonging to multiple domains.