We use the term ‘the moon’ to describe the rock that circles the Earth.
But it is not the only moon. Most planets have a rock (or two or three, or more).
Mars has two moons, Phobos and Deimos. ESA (European Space Agency) has a spacecraft which is now passing close to Phobos and it has taken some fantastic pictures.
Phobos is quite small and is not round, and each year it is pulled slightly closer to Mars. It is thought that it may hit Mars one day and cause an enormous explosion, in about 50 million years.