The Enderman is a mob that was introduced in version 1.8; The Adventure Update. The Endermen are depicted as 3 meter tall, black creatures with long arms and legs. They also have the ability to pick up and move blocks around.
Part of the Enderman's appeal is it's relation to Slenderman, a particularly terrifying internet meme. Notch specifically stated that Enderman was added to the game to give night an atmosphere of fear, and it generally accomplishes this due to its ability to teleport behind players and actually remove parts of a player's house while they're sleeping. Even with the recent downgrade, Endermen are still feared for their shock value.