Regenerative braking is a system that recovers some of the energy that would otherwise be lost as heat when slowing a car (or other vehicle) down. Cars with regenerative braking are typically hybrids that already have electric motors turning the wheels. These motors can be used in reverse as generators to slow the car and recharge the batteries. Alternative methods for storing the power include spinning a flywheel or pressurizing air.
Cars equipped with regenerative braking are equipped with regular braking systems as well, as regenerative braking systems often do not perform well at low speeds, and to help in emergency stops or in case of a failure of the regenerative braking system. The combination of the two different braking systems can be difficult to integrate, and on some vehicles can feel a bit unnatural. In some cases, often on fully electric vehicles, the regeneration kicks in as soon as the throttle is released in order to simulate the engine braking found on cars with gasoline engines. In most cases the regenerative braking system does most of the work, and drivers of vehicles with regenerative braking often report going the life of their vehicle without having to change the traditional brake pads or rotors.