Active Directory is the directory service for Windows Domain Networks. Active Directory allows for the control and monitoring of computers through a single domain controller. It allows a single user to sign in to any computer on the active directory network and have access to his or her stored files and folders in the server, as well as the local storage on that machine. This allows for any user in the company to use any machine that the company owns, without having to set up multiple users on a machine. Active Directory does it all.
Active Directory is very complex and securing it requires significant effort and years of experience.
It is used by many companies and is a vital skill to comprehend when attacking Windows.