A moon is a natural satellite of a planet. While a planet is a world which orbits a star, a moon is a world which orbits a planet. Moons tend to be smaller than planets, and orbit relatively close to them. While planets can be spread out and relatively far apart, a moon always sits close to its parent planet.
The environment and resources of a moon can differ greatly from those of their parent planet; Alioth, the planet where the Novark landed, has three moons. There are valuable resources on these moons that cannot be found on the planet itself.