From Wikipedia!

Reviewers are users who have the extra ability to check pending changes and configure the settings for the stable version which is selected and displayed.

Users in the reviewer group can:

  • Configure how the stable version is selected and displayed (stablesettings)
  • Edit semi-protected pages (autoconfirmed)
  • Have one's own edits automatically marked as "checked" (autoreview)
  • Mark others' edits as patrolled (patrolother)
  • Mark revisions as being "checked" (review)
  • Mark revisions as being "quality" (validate)
  • View recent changes patrol marks (patrolmarks)
  • View the list of pages with pending changes

For a list of all reviewers on Test Wikipedia, see Special:ListUsers/reviewer.