The term "forest" is used to describe a collection of AD domains that share a single schema for the AD. All DC's in the forest share this schema and it is replicated in a hierarchical fashion among them. The preferred model for Windows 2000 AD is to have an organization use a single forest that spans an entire enterprise.
While not an administrative block by themselves, forests are a major boundary in that only limited communication is available between forests. For example, it is difficult for a user in one forest to access a resource in another forest.
It is very difficult to integrate forests at this time because of potential problems reconciling schema differences between two forests.