A quick one
REEV = Range Extended Electric Vehicle.
The idea of a REEV is that an electric motor drives the car 100% of the time. REEVs plug in to your house and charge overnight or while you're at work (4-8 hours depending on batteries), and then the REEV drives purely on the more-efficient (though still not carbon-neutral) grid power for a set number of miles (generally betweem 20 and 60.)
After those 20 - 60 miles of driving, a small onboard generator kicks on to recharge the batteries and "extend the range" of the electric vehicle. This onboard generator can be anything that produces power: gasoline engine, diesel engine, ethanol engine, or even a hydrogen fuel cell. The vehicle remains as efficient as a hybrid even after the grid power is all used up because they still use regenerative breaking.
The engine (meaning that which does some kind of power production: gasoline/diesel combustion, ethanol combustion, fuel cell, etc) does not power the wheels, it charges the batteries. That means that forward motion is only being caused by an electric motor so they are, in all sense, electric vehicles (or EVs).
I think this is an interesting concept but I wonder: is this just another technology to "get us through the now" (such as the Prius - good for now but not forever) or is this our answer for the next century+? Something to think about...