In MediaWiki, the administrator can assign rights and privileges to the different users by making them members of a group. Each group is assigned certain rights by default, but the sysop, as well as the bureaucrat—who we will learn about shortly—can make changes to the rights that each group has. For instance, if a wiki was created as an HR manual, the administrator may not want all users to be able to create and/or edit pages. After all, not many companies would be too keen on an employee being able to change the dress code from business casual to pyjamas on Fridays!
The administrator can also create new groups should they choose. This could help out the sysop in the example above. Perhaps he or she wants to allow employees to discuss certain policies but not create pages or edit pages that contain corporate policies. While the administrator may know the wiki, they may not know the terminology used in the HR department. In this instance, they can create a new group...