The word "monument" comes from the Latin "monere" meaning "to remind" and traditionally means a building or a stucture (even if is only a gravestone) erected to remember or commemorate someone or something.
The word "memorial" is an object,institution or custom in memory of someone or something and its use is more fluid than monument.
However they may be officially named,I would say that all monuments are memorials but not all memorials are monuments except in a metaphorical sense.For example,the Prom concerts,the Nobel Prizes and the blue plaques around Britain are all memorials (to Henry Wood,Alfred Nobel and various famous people respectively),but are not usually described as monuments.
The Lincoln Memorial and Washington Monument are clearly both structures and both commemorative.They are clearly both monuments and memorials.The presence of bodies is irrelevant.
