Xylophone, vibraphone, marimba, glockenspiel, are all members of the percussion ensemble with definite pitch, and played with mallets.
Xylophone: set of wooden bars. Until some years ago, it had no resonators, but today most of them have.
Marimba: set of rosewood bars thinner, longer and wider than the xylophone's. The resonators have always been part of it.
Vibraphone: metal bars, uses vibrato or tremolo produced by an electrically driven series of fans located on top of the resonators tubes. It also uses pedal.
Glockenspiel: steel bars. Usually played with brass mallet.
(Taken from Adler's book on Orchestration).