Most hybrids on the road today use an electric motor to supplement a traditional gas or diesel engine. The very early versions from General Motors and Honda gain only what amounts to an "auto-stop" function which allows for the engine to turn off when the vehicle is stopped and turned back on again as the drivers begins to accelerate. The second generation hybrids (Toyota Prius, Ford Escape, and full size GM Trucks) began incorporating the electric motors inside the transmission and added larger batteries. This design allows some limited propulsion from the electric motors in addition to the auto-stop feature. None of these vehicles have the capability to charge the high voltage battery from a wall outlet in your garage.
Aftermarket companies have developed "plug-in" kits, mostly for the Prius which include a higher capacity battery and the ability to charge the high voltage battery from a standard electrical outlet. These "plug-in" hybrids offer an limited range where they can run on the electric motors only. The common bond with all of these hybrids is that the gas or diesel engine takes over for the electric motor when the high voltage battery gets low and propels the vehicle just as a non-hybrid, basically an internal combustion engine with and electric motor as a "helper" (Series Hybrid).
The next generation of "plug-in" hybrid is on the way in the form of the Chevrolet Volt, which GM is calling an "Extended-range electric vehicle" (or E-REV). The Volt breaks new ground in the fact that it is basically and electric vehicle with an internal combustion engine as the "helper" (Parallel Hybrid). There is no mechanical link between the combustion engine and the drive wheels. The Volt is always propelled by the electric motor; the combustion engine is used only to charge the high voltage battery via a generator. The Volt is designed to travel approximately 40 miles on electric power only, after which the operation of the gas engine is necessary to begin recharging the high voltage battery. In its current form the Volt uses a gasoline engine to power the generator which can provide enough electricity to keep the volt running with no need to stop for a recharge. Future Volt models could use a diesel engine or hydrogen fuel cell to power the generator.
The 40 mile range for electric operation only was chosen because more than two-thirds of Americans drive less than 40 miles a day. If those people plugged in their car to recharge it every night, they might never use any gasoline. And depending on local electric rates, that 40-mile recharge could cost less than a dollar.
The Chevrolet Volt has been 3 to 4 years in the making and is projected to be on sale by the end of 2010 (California only to start) with an estimated price in the $40K range. Look for production of about 10,000 vehicles the first year and 60,000 per year following.