What is a forest?

The Microsoft Windows 2003 Active Directory glossary defines a forest as “a group of one or more Active Directory trees that trust each other. All trees in a forest share a common schema, configuration, and global catalog. When a forest contains multiple trees, the trees do not form a contiguous namespace. All trees in a given forest trust each other through transitive bidirectional trust relationships. Unlike a tree, a forest does not need a distinct name. A forest exists as a set of cross-referenced objects and trust relationships known to the member trees. Trees in a forest form a hierarchy for the purposes of trust.”