There are different types of Hybrid Vehicle, the full hybrid, the micro hybrid and the mild hybrid.
The full hybrid is the most complex of all hybrid vehicles and are powered by both the electruc and combustion engine. No emissions are produced when the car is travelling at a lower velocity but when the car is travelling a greater speed the combustion engine takes over. The hybrid car is equipt with sensors to let the engine know what fuel source to use. Some hybrid vehicles are also adapted so that the braking system can actually recharge the battery so energy is almost never wasted.
The micro hybrid vehicle, also known as 'Stop-Start', is not a fully fledged hybrid vehicle but the Stop-Start technology assists with a reduction in emissions when you stop at traffic lights or brake and remain in the same position for an extended period of time.
The two types of mild hybrid vehicles are series and parallel. The series hybrid moves the car by using a battery which is charged either by a fuel based engine or when the car is slowing down. The parallel hybrid uses 'Stop-Start' technology and it also uses the electric motor to assist the engine during accelleration. The parallel mild hybrid requires both sources; combustion and battery, to function.