The term that really applies is HEV, which stands for hybrid electric vehicle. These are the most fuel efficient vehicles available for majority of us.
While actual configuration may vary due to the design intentions of the manufacturer, they generally combine a small (by regular vehicle standards) gasoline engine with an electric motor.
While the gasoline engine does most of the locomotion at highway speeds, when less fuel is required, it also is available to provide the extra power that the electric motor cannot provide or sustain for long.
The electric battery / motor combination can start the car moving in some instances, maintain systems without the gasoline motor having to idle at stops such as lights and in drive thrus, and can assist the gasoline motor as it propels the hybrid vehicle down the road. Not only is fuel being saved while the electric motor is maintaining vehicle systems in what would normally be idling situations, but this means that less pollution is being emitted into the air, usually within the confines of our most polluted areas … our cities.
During the rolling of the vehicle, particularly in coasting situations, the battery which provides the electric motor with power is recharged. When braking at slower speeds (in-town, stop-and-go), regenerative braking is generally used to stop the car rather than the brakes themselves. In regenerative braking, the electric motor actually changes is polarity, becoming a electric generator which recharges the battery. This change from motor to generator creates a reverse torque which is used to slow and stop the vehicle. At highway speeds, normal braking occurs.
At times, the charge within the battery may dip below the needed level, and at these times, the small gasoline engine automatically comes on to recharge the battery.
Even with hybrids, however, there are different vehicle types to meet the needs of the consumer. There are hybrid SUV's, Trucks, and small and mid-size sedans such as the Toyota Prius, which is one of the most fuel efficient hybrid vehicles available, getting on average about 50 MPG. Larger hybrid vehicles, such as SUV's and Trucks, tend to provide fuel economy in a relative sense. The fuel economy of these larger hybrid vehicles tends to roughly equal a gasoline powered sedan.