Active directory is a directory service / database which contains all the information related as part of domain. Like server / computer / users / groups and etc. All the entities are part of domain are call it as objects. So this ADS service will relate /integrate all the object in AD. This is same like our NDS in NT4X but NDS we cannot edit / update from anywhere. Since ADS allow administrator to edit the objects from any where we are calling this as a ACTIVE DIRECTORY.