The body that orbits a parent planet is called a moon, or more formally, a natural satellite.

Here are some other important points:

* Not all planets have moons. Some planets, like Venus and Mercury, have no moons at all.

* Moons can be different sizes. Earth has only one moon, which is relatively large. Other planets, like Jupiter, have dozens of moons, some of which are quite small.

* Moons can be made of different materials. Some moons are rocky, like our moon, while others are icy, or even made of gas.

Let me know if you have any other questions!