User summary from Wikipedia:
World of Warcraft: Wrath of the Lich King is the second expansion set for the massively multiplayer online role-playing game (MMORPG) World of Warcraft, following The Burning Crusade. It was announced on the first day of BlizzCon 2007, 3 August 2007.
Wrath of the Lich King was released on November 13, 2008, (nov 14 in Australia) selling 2.8 million copies within the first 24 hours of availability. This makes it the fastest selling computer game of all time, beating the record set by the previous World of Warcraft expansion The Burning Crusade, which sold 2.4 million within its first 24 hours.
This expansion requires the Burning Crusade to work, and adds the continent of Northrend, home of the titular Lich King, the ability to level up to level 80, and a new character "hero" class: The Death Knight, that can be any race and that starts on level 55. Players must have a high level character in order to create a Death Knight.
Source: Wikipedia, "World_of_Warcraft:_Wrath_of_the_Lich_King," available under the CC-BY-SA License.