Active Directory Trust relationship is a logical link which allows a domain to access another domain, or a forest to access another forest. Trusts which are created automatically are called as implicit Trusts and the trusts which are created manually are called as Explicit Trusts.
The following are the characteristics of Windows 2003 Trusts.
• Active Directory Trusts can be created manually (explicitly) or automatically (implicitly).
• Active Directory Trusts can be either transitive or non transitive. A transitive trust extends the trust relationships with other domains and a nontransitive does not allow the trust to flow to any other domains in the forest.
• Active Directory Trusts can be one-way or two-way.
Active Directory Trust Types
Parent-child Trust: Parent-child Trust is an implicitly established, two-way, transitive trust when you add a new child domain to a tree.
Tree-root Trust: Tree-root Trust is an implicitly established, two-way, transitive trust when you add a new tree root domain to a forest.
Shortcut Trust: Shortcut Trust is an explicitly created, transitive trust between two domains in a forest to improve user logon times. Shortcut Trust will make a trust path shorter between two domains in the same forest. The Shortcut Trust can be one-way or two-way.
External Trust: External Trust is explicitly created, non-transitive trust between Windows Server 2003 domains that are in different forests or between a Windows Server 2003 domain and Windows NT 4 domain. The External Trust can be one-way or two-way.
Realm Trust: Realm Trust is explicitly created transitive or non-transitive trust between a non Windows Kerberos realm and a Windows Server 2003 domain. This trust helps to create trust relationship between Windows Server 2003 domain and any Kerberos version 5 realm. The Realm Trust can be and one-way or two-way.
Forest Trust: Forest Trust is explicitly transitive (between two forests) created trust between two forest root domains. The Forest Trust can be one-way or two-way.