What is an LDAP Distinguished Name?

Distinguished name is a unique name for an entry in the Directory Service.

A directory service has entries that are similar to the UNIX file structure. For e.g., in Unix we have
/etc
/bin
/usr/bin
/usr/local

In a LDAP directory service, we can have the distinguished name:

cn=John Zukowski, ou=FAQ Guru, o=JGuru

Where:
o is Organization
ou is the Organizational Unit
cn is the Common Name

In the same LDAP service we can have the distinguished name:
cn=John Zukowski, ou=FAQ Member, o=JGuru

Both of these entries relate to same person (John Zukowski), but in different sub-trees (ou). One entry might have different permissions, rights, etc. when compared to the other. So the only way we can distinguish these two entries is by using their Distinguished names.