Access to various parts of your vBulletin installation is controlled by a rich and flexible permissions system. The basic unit which controls permissions is a usergroup. As you would expect, a usergroup is really just a group of any number of users.
Through this, you may control what this group of users is allowed to do on your board.
For example, perhaps you want only administrators to be able to post in a specific forum? Or only certain users to be able to attach files with their posts? Or maybe even allow a group of users to have their own user titles? These are all possible with usergroup permissions!