Active directory container is a built-in container that comes with AD and they cannot be altered to suit your organizational needs, without altering the AD schema. (Examples: Computers and Users containers).
An OU or Organizational Unit is a special purpose container created by the user and to which administrators can apply group policy. Group policy cannot be applied to a container. An organizational unit is a subdivision within an Active Directory into which you can place users, groups, computers, and other organizational units. You can create organizational units to suit your organization's functional or business structure. Each domain can implement its own organizational unit hierarchy.