A moon is a natural satellite that orbits a planet.
Moons vary in size across the galaxy, from asteroid size to nearly the size of a planet themselves. As a result, their gravitational fields can have differing effects on the planets they orbit. Luna's gravitational pull, for example, effects the tides of Earth.
Like planets, moons can vary in their appearence and make-up. Jerrado orbiting Bajor had a Class M environment that was capable of supporting life, while Luna was a desolate rock, until colonies were established in the 21st century and humans began adapting the landscape. (DS9 episodes: "Progress", "Valiant")