A distinguished name is a way to unambiguously identify objects through a directory service. Distinguished names are based on a hierarchy of object attributes. For example, in X.500, this hierarchy is expressed through object attributes such as:
A particular user, for example could be defined as CN=Doe, John, OU=auditors, OU=finance, O=Big Corp, DC=bigcorpinc, DC=com; C=USA. This would be a different user than someone of the same name in a different division: CN=Doe, John, OU=sales, O=Big Corp, DC=bigcorpinc, DC=com, C=USA.
In the examples above, "Doe, John" is a common name. "CN=Doe, John, OU=auditors, OU=finance; O=Big Corp, DC=bigcorpinc, DC=com; C=USA" is a distinguished name.
LDAP and Active Directory use distinguished names in a similar way.
See: RFC 2253