A domain controller is a single computer set up for client server operation rather than for the workgroup operation that smaller networks use. The domain controller can appear as a single server in a smaller network or as part of a forest on a larger network.
An Active Directory forest is a group of domain controllers that are working together to service user needs. All the members of the forest share data and update each other so that the loss of a single domain controller doesn't cause a complete failure of Active Directory or result in lost settings.
Active Directory is a special kind of hierarchical database that stores system settings, computer information, user information, application configuration, and a wealth of other information and statistics about your network.
Member Servers or just standalone servers can be promoted to domain controller. The Active Directory Wizard can help you install and configure components and enables you to provide directory service to network computers and users.
A server on a network - standalone or member - can function in a number of roles. As the needs of your computing environment change, you may want to change the role of a server. By using the Server Manager and the Add Roles Wizard, you can install Active Directory Domain Servers to promote a member server to a domain controller, or you can install individual roles or combinations of various roles, such as DHCP, WINS, and DNS.