Uber Monsters are special quest monsters added in v1.10 and v1.11. They can only be battled on the Battle.net Realms, and are difficult to find. They are also extremely difficult, with far higher stats and new abilities above and beyond those found on normal monsters in the game. The rewards they drop are far better as well, of course.
You'll encounter all but one of them as part of the Pandemonium Event. To learn about how to find Uber Diablo, visit his page.