Game worlds are a subset of all kinds of worlds, defined by a specific quality that other worlds lack. There is a problem to be solved, an area to be explored, a reward to be earned or a contest to be won. There is a kind of pressure when playing in a game, and a sense of risk. There is change, death, rebirth and a state of flux.
So in a sense the difference between a game world and a virtual world is one of motion. Game worlds are built for movement.