Early hybrid cars primarily used the electric motor for all power.
The engine would charge batteries from which the motor drew power, running only when needed to charge them back up.
More advanced hybrid cars reverse the process somewhat, using gasoline engine for primary power, but with greater efficiency that with conventional cars.
The modern role of the electric motor is to operate not only when the electric motor is used exclusively, but also when the gasoline engine requires extra power.
Also, in modern hybrid cars, instead of the just the engine charging the batteries, the motor acts as a generator during braking, using the momentum of the car to generate electricity
