Transitive Kerberos trust relationships between the trees. A common schema. A common global catalog. So all domains within a forest will share a common schema and catalog as well as transitive Kerberos trust relationships. The concept of contiguous trees combined in a forest makes it much easier to understand how search operations work within the forest or within the subtrees.
Security principals are still valid within the forest.
It is possible to form virtual teams by creating groups that have members from domains living in different trees within the forest. Trees can join a forest at any time. This helps to implement both grass-root deployments and enterprise merges. Metadata within each tree contains all information needed for a tree or a forest to work. Each container implements its own naming structure and, by doing this, its own replication topology.