Raid Bosses are classified as high level mobs that need a raid to defeat. Typically you'll only see these types of bosses at level 60 and higher and, on most of them, their level is depicted by a skull instead of the actual level. Prior to the release of The Burning Crusade, the majority of bosses required 40 people to take down, such as Onyxia but now, in Outlands, raids can and do consist out of 10 or 25 players, such as Gruul's Lair and Karazhan.
However, although each boss does have a recommended raid size, they have been and can be done with fewer players, depending on party make up, skill and gear. Each boss was originally tuned for raid group of the size listed at the of their introduction, but with the introduction of new equipment and the increase in the level cap, many of the fights are much easier now easier than they once were.