Hybrid Electric Vehicles are vehicles that use two different power sources to propel the vehicle, one of which is usually electricity stored in a battery pack.

The second power source can be Diesel, Gasoline, Natural Gas, Propane, Wood, Coal, or any other power source.
The most common are Diesel and Gasoline.

There are two common types of Hybrid electric Vehicles, Parallel Hybrids and Series Hybrids.


Parallel Hybrids:
In a Parallel Hybrid the electric motor and the internal combustion engine are both connected to the wheels mechanically. In some cases the electric motor can power the car independent of the ICE and visa versa (eg. the Toyota Prius), in other cases the electric motor merely assists the ICE when it requires more power (eg. the Honda Insight, most mild hybrids). Since the electric motor is generally smaller in parallel hybrids, the top speed is usually limited in the electric only mode.



Series Hybrids:
In a series hybrid the electric motor is the only thing that can move the car forward. A series hybrid uses the electric motor as the means of movement and the internal combustion engine is used as a generator that makes electricity. This electricity can either be used right away by the motor, or be stored in the battery pack of the vehicle to be used later. This type of hybrid may also be called a Range Extended Electric Vehicle, especially if it is able to operate without the use of the generator until the battery pack is low (eg. GM's Volt)


Plug-In Hybrid Electric Vehicles:
Plug-In Hybrid electric vehicles can be either a Parallel or Series design, though generally it is more suited to series hybrids. The only difference between a plug-in and a conventional hybrid is that you can plug your vehicle into an electrical outlet to charge the battery of the vehicle. This is done in order to minimise the use of the internal combustion engine while on the road.


Conclusion:
Hybrid electric vehicles may be considered a mid-point between fully electric vehicles and fossil fuel powered vehicles. Fully electric vehicles currently have limits on their range due to battery capacity, they also are limited to consumers because the price of the batteries is high. A Hybrid dramatically reduces the battery pack size required to have a usable range, thus reducing the initial cost of the vehicle. Thus many view hybrids as a good compromise between an EV and a traditional ICEV until the price and energy storage of batteries improves.