Active directory is a centralized directory system used on Microsoft networks. Active Directory is organized with forest, trees and domains. Within the domains you have organizational units. Inside of the organizational units you can apply group policies. So with Active Directory you only need one log in to access all of your companies resources.
For example, when you log in to the active directory it will authenticate you are the print server, web server, and mail server, etc. This comes in handy because the different servers may require you to change passwords at different times, but in this case Active Directory will handle this. It is a centralized resource.