These are used interchangeably because many believe they are the same thing. In actuality, mass describes matter and its volume (or how much space it takes up)- whereas weight describes the extent of gravity's pull on an object.
For example, in space, astronauts have been able to float freely without the use of a jet-pack or some other mechanical device. This is not because they have suddenly lost mass- they still have as much matter and volume as before. But they have left the confines of Earth, which has significantly reduced the effects of gravity, thereby lessening their "weight" and enabling them to float.
