I don't know the difference. I am currently enjoying my Meguiar's Gold Class liquid wax. While I don't like to mess with what works well, I just feel that paste is more of a "real" wax than liquid.
A long time ago Consumer Reports did a study on different waxes. Their findings showed that liquids performed no worse (or better) that paste. They also reported that paste was still manufactured by many vendors because many consumers thought it was "more of a real wax than liquid".