A planetary ring is a ring made of dust and other particles orbiting around a planet in a flat-disc shaped region. The gas giants Jupiter, Saturn, Uranus and Neptune are the only planets known to have rings, with the rings of Saturn being the most spectacular system of rings.
The rings sometimes have "shepherd satellites". These moons orbit either in between ring gaps or the outside edge of a ring. The moons' gravity maintain the rings current shape with a well defined edge. Any material that drifts closer to a Moon's orbit, it can either move back into the body of the ring, deflect out into space, or is added onto the Moon's surface.