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 we deploy in an Active Directory forest is called the forest root domain and it remains the forest root domain for the life cycle of the AD DS deployment.
The forest root domain contains the Enterprise Admins and Schema Admins groups. These service administrator groups are used to manage forest-level operations such as the addition and removal of domains and the implementation of changes to the schema.
Selecting the forest root domain involves determining if one of the Active Directory domains in your domain design can function as the forest root domain or if you need to deploy a dedicated forest root domain.