Multiple domains form trees within a forest. A tree can be defined as a hierarchical arrangement of domains within a forest. A tree is established when a child domain is added to the hierarchy under an existing parent domain. When you add a new domain as a child domain, it inherits a portion of its namespace from its parent. When you create a new tree within a forest you are establishing a new namespace.
You can create a new tree within a Windows Server 2008 forest using the steps below: