A hybrid vehicle (HV) is a vehicle using an on-board rechargeable energy storage system (RESS) and a fuelled power source for vehicle propulsion. The HV pollutes less and uses less fuel. The different propulsion power systems may have common subsystems or components. The HV provides better fuel economy than a conventional vehicle because the engine is smaller and may be run at speeds providing more efficiency. Other techniques may be used to recover or reduce waste energy (such as regenerative braking and shutting down the combustion engine).

The second question : who is the greenest?  From the last link you will find that now Toyota Prius is the cleenest in the test of greenhouse gas emission rating shows the annual amount of greenhouse gases emitted by the selected vehicle in relation to the lowest and highest amounts emitted by other vehicles.

http://en.wikipedia.org/wiki/Hybrid_car

http://en.wikipedia.org/wiki/Types_of_hybrid_vehicle

http://en.wikipedia.org/wiki/Plug-in_hybrid_electric_vehicle

http://www.fueleconomy.gov/feg/hybrid_sbs_cars.shtml