What are the types of Signalling Systems and How does CBTC (Communication based train control) works?

Railway signalling is a system used to direct railway traffic and keep trains clear of each other at all times. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormous weight and inertia of a train, which make it difficult to quickly stop when encountering an obstacle.

Can you please name the types of Signalling system used in Metro Rail System? Please also explain how does CBTC system works?