Before we jump into how gas-electric cars work, let’s get some terminology straight so we can better communicate about them.  You’ll likely have seen these terms in news media and literature before.

Full Hybrids

Full Hybrids have both a combustion and electric propulsion system (e.g. a gasoline and electric motor) and can operate on one, the other, or both, depending on the hybrid drivetrain type.

Mild Hybrids

Mild Hybrids are vehicles that rely on combustion fuels to move the vehicle, but which stop the engine when the vehicle is stopped (anti-idle) and they often store braking power as electricity (regenerative braking) to power vehicle components and save the engine power required to power accessories.  These are the least efficient of the hybrid vehicle types, but they are a step in the right direction and can give fuel efficiency bonuses of 10-15% in most combustion cars.

Parallel Hybrid

Parallel Hybrid vehicles are those that can move using both electricity and combustion through the vehicle’s transmission or gearbox.  So, while the two propulsion sources are separate, they operate the same gearbox and drivetrain.  The illustration below, from Wikimedia, shows how this works.

Series Hybrid

Series Hybrid power trains are those which allow both propulsion systems (electric and combustion) to act separately.  The electric propulsion system is the only means of moving the vehicle while the engine acts as a generator to provide electricity for the motor.  A Range-Extended Electric (REEV), Plug-in hybrid electric (PHEV), or similar vehicle is a series hybrid.

Series-parallel hybrid

Series-parallel hybrid, or Power-split, is a combination of the Series and Parallel powertrains.  These allow power paths from the motor or engine to power the wheels.  Usually, they will have one axle run by the engine and another by the motor.  For instance, the Audi hybrid system usually puts the engine at the front, powering the front wheels, while the motor is in the rear, powering the rear wheels.  These are becoming more common in PHEVs because of their simplicity and redundancy.  This power train is illustrated below in an image from Wikimedia.