What is organization unit?

Answer 1

An organisational unit or OU for short is something you are going to have be familar with if you plan on doing much work with Active Directories (AD).

Say in your AD you have 200 users, you could then put these users into OU's such as 'accounts', 'managers', 'engineers', 'database users' etc...

Now you can say everybody in accounts needs to be able to access print on this printer. So now you just apply permission to the OU instead of ever single user. Or you could say all engineers need to be allowed to have remote access, so again just apply the permission to the OU.