We know of nearly 150 natural satellites of the Sun’s collection of planets.  Some, such as Titan, are nearly as large as a planet. The four big moons of Jupiter discovered by Galileo in 1610 are worlds in their own right. The smallest moons are not much more than large rocks, captured from somewhere else in the solar system.  The two places from which moons tend to be captured are the asteroid belt between Mars and Jupiter, and the Kuiper belt, out beyond Neptune. These satellites of the planets come in a wide variety of shapes and sizes.
Carpe Caelum Planetary Astronomy