Active Directory is Microsoft’s version of an LDAP server. It’s a specialized type of database that stores information about users and network resources and security information about those users and resources.
Multiple AD servers on a large corporate network can be linked so that changes to one AD server get propagated to all of the other AD servers. This allows each geographical location to have its own copy of user accounts and network resource listings so that programs that need to query for them can query a local machine, rather than sending query requests out over the larger WAN, which is generally a very slow operation.