What is Active Directory ?

 Active Directory:

 

Introduced first by Microsoft in window 2000 (operating System) . It is a centralized database which has information about all the     object. ie  users, computer resources like printer etc. in an organization      network  

Today most window server (OS) include active directory.  

Domain Controller: 

If an active directory domain service (ADDS)  is running in a server is called domain controller . It authenticates and authorizes all users and computer in a window domain type network assigning and enforcing security policies for all computer and installing or updating software.  

Example of AD: 

When a user want to login to a computer which is part of window domain. Active directory check first whether the email and password match with the active directory server email and password or not. If it does match access provided. Other wise login avoided by an error, or warning.