Why are some planets surrounded by rings?

If you take Saturn with its rings, Saturn is not a perfect sphere. This is because it's spinning and its mass gets thrown outwards. This makes it a bit wider than it is tall. Because of this, there's a concentration of mass around the centre, which means that stuff is more attracted to the area at Saturn's equator than anywhere else. So even if you started of with a spherical sphere of debris around the planet, then eventually as they bump into each other and jostle each other, it will all settle down into a ring. This extra mass around the centre provides that extra bit of pull and forms rings instead of a sphere. This is exactly the reason that the planets in our solar system form the planar structures that they do. This is also why moons tend to be in the equatorial plane. It's all the same theory.