Users, groups (domain local, global, universal), OUs and delegation of control; group managed service accounts and the KDS root key

Users, computers and groups are the everyday objects of AD. Organizational units (OUs) are containers you create to organize those objects. OUs matter for two reasons: you link Group Policy Objects (GPOs) to them, and you delegate administrative control over them. The built-in Users and Computers containers are not OUs, so you cannot link GPOs to them; most organizations build an OU structure by location, department or object type and move objects into it.