When you step on your brakes, you are converting kinetic energy to heat. Kinetic energy can get you to the grocery store. Heat can not. If you could figure out how to harness the heat or if you could figure out how not to brake at all, you could save all sorts of energy..
Luckily, some really smart engineers are working on braking systems that capture the kinetic energy that braking dissipates as heat. These systems are called regenerative braking.
Regenerative braking systems are most common in hybrids like the Toyota Prius and in fully electric cars like the Tesla Roadster. Here is how conventional four wheel disc braking systems work: when a driver depressed the brake pedal, a piston in the master cylinder forces brake fluid to close calipers which presses brake pads against steel rotors. This friction slows your vehicle as it converts kinetic energy to heat. With regenerative braking, stepping on the brake pedal causes the electric motor that powers the vehicle to run backwards causing the motors to act as a generator. Kinetic energy is converted into electricity and stored in the batteries. Because this type of system can not produce sufficient stopping power for all situations, hybrids and electric vehicles also have conventional friction brake systems.