Natural satellites come in all sizes, from small rocks to worlds with oceans and atmospheres.
A moon is a natural object that orbits a planet, a dwarf planet or an asteroid. In the Solar System there are hundreds of known moons, and new ones are still being discovered around the giant planets.
Mercury and Venus have no moons, Earth has one and Mars has two. Most moons belong to the giant planets, and some of them are among the most interesting places in the Solar System.