Active Directory has forests and trees which are ways of representing multiple domains. This video looks at how domains sharing the same namespace are considered a tree. Domains in separate namespaces are considered separate trees in the same forest.
When you have multiple domains in the same namespace (e.g., ITFreeTraining.com, west.ITFreeTraining.com, and sales.ITFreeTraining.com), they are considered to be in the same tree. The tree also supports multiple levels of domains. For example, you could have west.sales.ITFreeTraining.com and east.ITFreeTraining.com in the same tree.
A forest is a collection of one or more domains which may have one or more trees. What makes a forest unique is that it shares the same schema. The schema defines what and how Active Directory objects are stored. The schema defines the database for the whole forest but it should be remembered that each domain in the forest has its own copy of the database based on the schema.
Parent and child domains are automatically linked by a trust. Users in different domains can use these trusts to access resources in another domain assuming that they have access. Trees in the forest are linked together via a trust automatically. This ensures that any users in any domain in the forest can access any resource in the forest to which they have access.
In order for users to find resources in any domain in the forest (remember that each domain has a separate database), Domain Controllers can be made into Global Catalog Servers. A Global Catalog Server contains partial information about every object in the forest. Using this information, the user can conduct searches.