Microsoft Knowledge Base Article 308194 contains the following summary:
This article describes how to create organizational units.
Organizational units are Active Directory containers into which you can place users, groups, computers, and other organizational units. You can create organizational units to mirror your organization's functional or business structure. Each domain can implement its own organizational unit hierarchy. If your organization contains several domains, you can create organizational unit structures in each domain that are independent of the structures in the other domains.
This article explains how to create domain organizational units.
An organizational unit is the smallest scope or unit to which you can assign Group Policy settings or delegate administrative authority. By using organizational units, you can create containers in a domain that represent the hierarchical, logical structures in your organization. You can do this to help you manage the configuration and use of accounts and resources based on your organizational model.
Organizational units can contain users, groups, computers, printers, and shared folders, as well as an unlimited number of other organizational units, but cannot contain objects from other domains.