Active Directory is a directory service created by Microsoft for managing Windows environments.
It is used for:
- A centralized Authentication/Authorization source for Kerberos and NTLM protocols.
- Providing a single authentication/authorization domain for member servers and workstations.
- Providing LDAP services.
- Providing centralized configuration methods of Windows workstations and servers through Group Policy and other methods.
- Multi-site replication of directory-database data.
Non-Windows support is generally provided through the Samba package on POSIX operating systems (Linux, OS X, BSD, Solaris, etc). All modern Samba releases allow machines to join a domain as if they were a Windows machine.
For a list of ports which must be open in a firewall for AD to work properly, see What firewall ports need to be open for Active Directory?.
Some useful links are below
http://blogs.technet.com/askds - Official blog from Microsoft engineers supporting active directory http://blogs.technet.com/activedirectoryua/ - Official blog from technical writers at Microsoft for AD http://blogs.msdn.com/b/active_directory_team_blog/ - Official blog from product group for AD and other related technologies