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 the 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