Superusers are regular users who’ve been hand selected by the Foursquare staff to help maintain the venue database.
There are three types of superusers, Level 1, Level 2, and Level 3. Level 1 superusers have the ability to edit any venue, including name, address, map pin location and tags. They can also suggest duplicate venues to be merged and venues to be deleted.
Level 2 superusers have all the abilities of a Level 1 superuser, but they can also complete merges suggested by other superusers, add categories to venues and close venues. To merge venues, they use a special interface that shows both venues to be merged. They can choose which information to keep from each venue page, but they cannot choose which map pin should be kept. This is why it’s important to choose the location with the best map pin when suggesting venues to be merged.
Level 3 superusers can create and remove venue aliases. They can also lock venues so they can only be edited by other Level 3 superusers. This promotion is only awarded very rarely.
UPDATE (1/24/10): I’ve made a few edits to reflect some of the changes that have taken place for superusers since this post was last edited in May.