What is an Active Directory (AD) ?

The Microsoft Windows 2003 Active Directory glossary defines an Active Directory as “a structure supported by Windows 2003 that lets any object on a network be tracked and located. Active Directory is the directory service used in Windows 2003 Server and provides the foundation for Windows 2003 distributed networks.” A directory service “provides the methods for storing directory data and making this data available to network users and administrators. For example, Active Directory stores information about user accounts, such as names, phone numbers, and so on, and enables other authorized users on the same network to access this information.”
The AD, or Active Directory, is a database based on the LDAP (Lightweight Directory Access Protocol) standard, which makes the information contained within the AD easily available to other applications across different platforms. The AD contains user accounts, computer accounts, organizational units, security groups, and group policy object - all of which have a unique name and a unique path. All unique objects in the AD use a domain contained within the AD as a means of authentication.