An album of one artist or bands' songs covered by other artists as a "tribute". Many times, these tributes are of dubious quality, comprised of relatively unknown musicians piggy-backing off of the covered artists' fame.
Although the tributes are usually of the same genre, it is often the case that they are of a different genre entirely. (For instance, D.O.A. - A Punk Tribute to Metallica, or Powerslaves: An Elektro Tribute to Iron Maiden.) Some companies, like Cleopatra Records, crank out tribute albums instead of the traditional "discount sampler" albums, to draw attention to themselves and their artists.
Although rare, occasionally a tribute album will gain notoriety for its innovative and unusual take on its material. A good example of this is Apocalyptica's Plays Metallica by Four Cellos.