Active Directory is the database that holds all directory information such as information of users, computers, groups, other objects, and the objects that users can access. It also includes other network components. Also it provides centralized security, Authentication & Management services.
The Active Directory database is stored on the server’s hard disk by means of the Ntds.dit file in Ntds folder of system root. The file has to be stored on a partition with NTFS file system.
Domain controllers manage domains; each domain controller hosts a read-write copy of the Active Directory. When changes are made to the data store on one domain controller, these changes are automatically replicate to all domain Controllers, this provides fault tolerance.