RG is an old, long-forgotten military term meaning "radio guide". The number distinguishes different cable specifications; but they are randomly assigned, so don't look for some sort of ordering in numbers. Really a lot of coaxial cables have been made over the years but the only thing most people have to worry about is RG6 Cable and RG59 Cable, so those are the two types we're going to focus on here.
The RG6 Cables are best for high bandwidth or high frequency applications such as Internet, Cable TV and Satellite TV signals.
The RG59 Cable comes with a smaller conduction. It is also thinner and less shielded. It is only recommended in low bandwidth and low frequency applications such as CCTV System installations. The Siamese Coaxial Cable consists of RG59 Cable combined with a 2C Power Cable.