forest trust

http://technet.microsoft.com/en-us/library/cc754612.aspx

transitive trusts:

• Shortcut trust: A transitive trust between a domain in the same domain tree or forest that shortens the trust path in a large and complex domain tree or forest.
  
• Forest trust: A transitive trust between a forest root domain and a second forest root domain.
  
• Realm trust: A transitive trust between an Active Directory domain and a Kerberos V5 realm. For more information about Kerberos V5 realms, see Kerberos V5 authentication (http://go.microsoft.com/fwlink/?LinkId=92699).

nontransitive trusts:

• External trust: A nontransitive trust between a Windows Server 2008 or a Windows Server 2008 R2 domain and a Windows NT domain or a Windows 2000 domain, Windows Server 2003 domain, Windows Server 2008, or a Windows Server 2008 R2 domain in another forest.
  
• Realm trust: A nontransitive trust between an Active Directory domain and a Kerberos version 5 (V5) realm. For more information about Kerberos V5 realms, see Kerberos V5 authentication (http://go.microsoft.com/fwlink/?LinkId=92699).