What are the e-Tron's regenerative braking systems and how do they work?

I've been hearing a lot about electric cars and their regenerative braking systems - specifically with the e-Tron. Can someone explain how these systems work and how they are different from traditional braking systems?

The Audi e-Tron uses a two-stage regenerative braking system, allowing the driver to choose the level of energy regeneration. This means that when you lift off the accelerator pedal, the car will use its electric motor to slow the car down and convert that kinetic energy into electricity, which is then stored in the battery. You can choose between three different levels depending on your driving style and environment, with the higher levels providing more aggressive energy regeneration.