A moon is defined as a natural satellite that orbits a larger body--such as a planet--that, in turn, orbits a star. The moon is kept in its position both by the gravity of the object that it circles, as well as by its own gravity. Some planets are orbited by moons; some are not. Some dwarf planets--such as Pluto--possess moons. In fact, one of Pluto's moons, named Charon, is almost half the size of Pluto itself, and some planetary scientists think that Charon is really a chunk of Pluto that was torn off in a disastrous collision with another object very long ago. In addition, some asteroids are also known to be orbited by very small moons.
Popular culture has tried to extract maximum leverage out of the mysterious symbolism associated with the full moon. Modern fables have produced creatures like the were-wolf, an otherwise normal man who apparently becomes a wolf when the moon is full. A full moon has strong suggestions of pure and predominantly platonic, love.
Most of the moons of our Sun's family circle the quartet of large gaseous planets located in our Solar System's outer limits: Jupiter, Saturn, Uranus and Neptune. The four solid inner planets--Mercury, Venus, Earth and Mars--are almost entirely moonless. Earth is the only inner planet that hosts a large Moon, while Mars sports only a pathetic duo of misshapen little Moons (Phobos and Deimos), that are either captured asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter, or are instead the outcome of a primordial collision between Mars and a large protoplanet.