The World of Darkness is a games setting based on our own earth, but darker and more sinister, being filled with monsters. This world is described in 5 interlinked games from White Wolf:
Each of these self contained games explores one monsterous race in the World of Darkness, and their interactions with humanity. The twist that has made the WOD games the most popular roleplaying system in recent years is that the players play the monsters.
The fist of the games, and by far the most popular is Vampire, in which players take the part of immortal bloodsucking creatures from nightmare. Similarly in Werewolf they play werewolves, in Wraith ghosts, in Mage magicians and in Changeling faries. While some of the games, most notably Mage and Wraith have been criticised as being hard to play and imbalenced the system as a whole has changed the face of roleplaying, generating huge numbers of other games attempting to copy the dark and forbidding style of the games. (Rather reminiscant of Cyberpunk).
Part of Vampire's attraction is of course that Vampires are hip and trendy at the moment, especially amongst students, but at the same time many gamers see it as a real and interesting challenge to play an eveil creature forever aware of the loss of its own humanity. The system has spawned many spinoffs, including a Fox TV series, Kindred.