Hybrid cars are part battery-electric and part conventional cars. They use of a temporary energy storage device (usually a battery), which enables the main engine to be operated at close to its maximum efficiency.
There are two types of hybrid car, those that use an engine to produce electricity and then use an electric motor for propulsion (these are less common.) And those in which the conventional engine or electric motor can be used to drive the vehicle. Batteries charge and act as a storage device to power the electric motor, usually working when the vehicle is travelling at low speed or in traffic, making them ideal for city driving. A petrol engine, usually a small efficient unit then powers the car when more power is needed such as at higher speeds allowing the combustion engine to only operate at its more optimum efficient speeds. A number of manufactures have announced plans for diesel-electric hybrids which could be available a soon as 2008.
The advantage of hybrid cars is their increased fuel economy and lower CO2 emmissions.