Although "full" hybrid technology is understandably receiving lots of buzz in recent years, especially now that the auto industry is undergoing such upheaval, mild hybrids are also getting some attention lately. Despite the name, mild hybrids aren't technically hybrid cars. They're actually conventional vehicles that use internal combustion to turn gasoline into energy and rotational motion, propelling a vehicle forward. There is no electric motor in a mild hybrid to move the car, and although there's a battery, it serves a different purpose than a battery does in a full hybrid.
The battery in a mild hybrid is still very important, but its main purpose is as a part of the idle-stop system that shuts off the gasoline engine when a mild hybrid is at rest, coasting or slowing down. For instance, a stop at a red light at an intersection will cause the engine to cut off. When the car is idle, no gas will burn inside the engine. Once the light turns green and the driver applies pressure to the gas pedal, the engine should switch back on seamlessly, as though it hadn't been turned off at all.
There are essentially three main parts involved in an idle-stop system: the gasoline engine, an electric starter/generator and a battery. The transfer of energy works in that order, both forwards and backwards -- it just depends on what state the car is in. When the car's engine is on and you're just about to brake, stop-start systems use regenerative braking, where rotational energy from the wheels turns the electric generator and creates electricity. The generator sends electricity to the battery where it can be stored for later use. When the driver applies the brakes, however, the generator shuts off the gasoline engine. Pressing the accelerator pedal starts the engine once again by taking the stored energy from the battery and running it through an electric starter.