Beware - ThisIsNotAmerica - if you want to search Google for OU, you might want to spell it 'Orgizational Unit'.
An Organizational Unit object (commonly referred to as an OU) is a Container? object in LDAP that is used for keeping similar objects together.
If you have LDAP as a tree inside your company (like ActiveDirectory) then it makes good sense to put different departments, sites or teams inside seperate OU's.
OUs primarily satisfy the need to delegate administration to separate groups of administrators. Although there other possibilities for the use of OUs, this type of administration delegation is, in reality, the primary factor that exists for the creation of OUs.