In Microsoft networking, a domain is a basic security unit, with a unique name, which provides access to the centralized user accounts and group accounts maintained by the administrator of the domain. Each domain has its own security policies and security relationships with other domains. Domains can span multiple physical locations. As the name implies, the network consists of one domain to which all user accounts and resources belong. In the single master domain model, the network is structured into two or more domains, with all user accounts placed in one domain, called the master domain.
All users log on to the master domain.
Other domains, which can hold computer accounts, shared files, printers, and other network resources, are called resource domains. The advantage of this is that user accounts can be managed centrally, while departments or divisions can manage their own resources. The multiple master domain model is an extension of the single master model. In this case, there are two or more master domains into which user accounts are placed. This is a way of scaling the master domain concept to a large network, in which there are too many user accounts to fit into a single master domain.