So now what is up with the manual? it kinda sucks. The game is very intuitive, but it seems that there is a lot of info left out. Moons for instance (which you mentioned in your post) are mentioned nowhere... at least that I can find. What do they do? The answer to that is not so important as where am I supposed to read about them? I'll have lots and lots of basic questions.
Moons are simply small planets in the same sector as a larger one. There is no special in-game definition of a moon, but if you saw one, that's what you'd automatically call it