Active Directory is uses the Lightweight Directory Access Protocol (LDAP) as its primary access protocol. LDAP runs over TCP/IP, and defines a way to reference and access objects between an Active Directory client and server. Under LDAP, every object has a distinct Distinguished Name, and this name distinguishes the object from every other object in Active Directory, while also telling us where the object exists. The two main components of a distinguished name are a CN (common name) and a DC (domain component). The common name identifies an object or the container in which it exists, while the domain component references the domains within which the object exists. For example, a distinguished name could be as follows:
CN=Dan DiNicolo, CN=Users, DC = 2000trainers, DC=com
In the above example I have a user called Dan DiNicolo, who exists within a container called Users, in the domain 2000trainers, which is a subdomain of com. The distinguished name of an object must be unique within a given Active Directory forest (more on forests in a bit).
While a distinguished name tells us about the complete context of an object, a relative distinguished name uniquely identifies an object within its parent container. For example, if I were searching within the Users container, the relative distinguished name of the object I identified above would be Dan DiNicolo.