An Active Directory tree is a collection of Active Directory domains that begins at a single root and branches out into peripheral, child domains. Domains in an Active Directory tree share the same namespace. An Active Directory forest is a collection of Active Directory trees, similiar to a real world forest.
Following figure explains an Active Directory Tree.
An Active Directory domain tree forms a contiguous namespace. The domains in the Active Directory tree share a boundary with each other. When a new domain is added under another domain in an Active Directory tree, a parent-child relation is formed between the existing domain and the newly added domain.
No two tree in an Active Directory forest can share the same name space. For example, the first tree in the above Active Directory forest is omnisecu.com and the second tree in the above Active Directory forest is omnitech.com. The child domains of omnisecu.com are sales.omnisecu.com and rd.omnisecu.com. The child domain of sales.omnisecu.com is sw.sales.omnisecu.com