Active Directory (AD) is a directory service for Windows network environments. It allows an organisation to store directory data and make it available to the users in a given network. AD has a distributed hierarchical structure that allows for the management of an organisation's resources such as users, computers, groups, network devices, file shares, group policies, servers, workstations and trusts. Furthermore, it provides authentication and authorization functionality to Windows domain environments.
Essentially, AD is a large database of information which is accessible to all users within a domain, irrespective of their privilege level. This means that a standard user account can be used to enumerate a large portion of all AD components.