OU's have many of the attributes of an NT 4 domain. However, instead of requiring server resources to create and support, they are a logical construct within the Active Directory so an OU does not have to support and maintain a domain controller.
OU's are created by an administrator of an AD domain and can be freely named (and renamed). The OU can then be populated objects of many types including computers, groups, printers, users and other sub-OU's.
The real power of an OU is that once it is established, the administrator of its "parent" can delegate administrative authority -- in total or in part -- to any user or group that is in the AD.
When this happens, the designated user/group gains complete administrative authority over all objects in their OU and thus has all of the rights and abilities that a Windows NT domain administrator would have as well as some new ones such as the ability to further segment their OU into sub-OU's and delegate authority over those sub-elements as they see fit.