Active Directory is a centralized authentication and directory service based around standards such as the Lightweight Directory Access Protocol (LDAP) and Kerberos. It stores information about user accounts, groups, distribution lists as well as information for directory enabled applications such as Microsoft Exchange Server.
The forest acts as a security boundary for an organization and defines the scope of authority for administrators. By default, a forest contains a single domain, which is known as the forest root domain however additional domains can be created in the forest.