Different users have access to different functions of the site. While anyone can do most things on the site, including reading and editing, rollbackers can access a few additional functions.
Anyone can revert vandalism and bad-faith edits, but it takes a couple of clicks in the page history to get it done. Users with the "rollback" permission are able to undo bad edits with one click, by using the rollback link on diff pages, user contributions list, or the list of recent changes.
It is important to note that admins (including bureaucrats) already have rollback rights by default. Users with rollback rights are just users who are not necessarily an admin.
Who are this wiki's rollbackers?
Rollbackers (users with Rollback rights) can be seen via Special:ListUsers, but they do not by default have any special title that appears on their userpage.