Hybrid cars use a combination of two power sources to drive the car, a petrol or Diesel combustion engine and an electric motor. Batteries act as a storage device to power the electric motor, usually working when the vehicle is travelling at low speed or in traffic. The petrol/diesel 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
The combustion engine will also recharge the battery cells along with the technology of regenerative braking. Regenerative braking is when kinetic energy from the motion of the car is used to generate electric power through the act of braking to charge the batteries, therefore hybrid cars do not need to be plugged into an external power supply. Unless you have a Plug in Hybrid Electric Vehicle (PHEV). This is basically a Hybrid car with a larger battery bank enabling the driver to charge the car over night, thus getting longer out of electric driving whilst having the backup comfort of a combustion engine.
This combination of battery power and internal combustion engine produces less pollution and CO2 as no gases are released when the electric motor is running. A number of Car manufacturers are competitively introducing different technologies to improve the efficiency of Hybridisation which is unquestionably the way forward in combustion powered motoring.