A domain controller is a server that manages network security and user access within a computer network. It stores information about the network's users, computers, and other resources, allowing administrators to control who can access what. This is essential for maintaining security and organization in larger networks. When a user logs into a network, the domain controller verifies their credentials against its database. If the information matches, the user gains access to the network and its resources. This process is crucial for systems using Active Directory, which is a directory service developed by Microsoft for managing permissions and access to networked resources.