Raids:
Raids are basically just very long Dungeons with a very large group of players to defeat some very high level Beasts. The higher the Raid lvl requirement's the harder it is to complete, but if you succeed you will sometimes get Rare Mounts. For example you will need to obtain the "Glory of the Icecrown raider (10 player)" achievement to get the rare mount The "Bloodbathed Frostbrood Vanquisher" Mount.