WHAT IS DOMAIN ?

Domain:

A domain is a logical grouping of networked computers in which one or more of the computers has one or more shared resources, such as a shared folder or a shared printer, and in which all of the computers share a common central domain directory database that contains user account security information. One distinct advantage of using a domain, particularly on a large network, is that administration of user account security for the entire network can be managed from a centralized location. In a domain, a user has only one user account, which is stored in the domain directory database. This user account enables the user to access shared resources (that the user has permissions to access) located on any computer in the domain

Active Directory domains can hold millions of objects, as opposed to the Windows NT domain structure, which was limited to approximately 40,000 objects. As in previous versions of Active Directory, the Active Directory database file (ntds.dit) defines the domain. Each domain has its own ntds.dit file, which is stored on (and replicated among) all domain controllers by a process called multimaster replication. The domain controllers manage the configuration of domain security and store the directory services database. This arrangement permits central administration of domain account privileges, security, and network resources. Networked devices and users belonging to a domain validate with a domain controller at startup. All computers that refer to a specific set of domain controllers make up the domain. In addition, group accounts such as global groups and domain local groups are defined on a domain-wide basis.