Various degrees of hybrids

 Even after 10 years, hybrid-electric vehicles are still foreign to large parts of the car-buying public. People aren’t sure if they’re worth the money, they worry that the battery may need replacing, and some still don’t understand that you don’t need to plug in a hybrid.
Well, brace yourselves. Things are going to get vastly more complicated. And one cause of confusion is the difference between a “plug-in hybrid” and cars like the 2011 Chevrolet Volt, which plugs in and has both a battery pack and a gasoline engine.
Hybrid: mostly gasoline powered
As we commonly use the term “hybrid,” it refers to a car with a combustion engine that derives some part of its motive power from electricity. In cars like the 2010 Toyota Prius, that energy is recaptured via regenerative braking.
In the future plug-in version of the Prius, some of that energy will also come from plugging the car into the electric grid. So, if you can plug it in, it becomes a “plug-in hybrid,” or PHEV. That may give it 10 or 12 miles of electric range, but only under a limited range of circumstances (low speeds, light loads, warmed-up engine).
The 2011 Chevrolet Volt, on the other hand, is an electric vehicle, pure and simple. It’s an EV with a very short range on battery power, mind you: just 40 miles. But for those 40 miles,  it runs exclusively on battery power as an electric car.
Volt: electrically powered, always
After that, the gasoline engine in its Voltec drivetrain switches on, but it doesn’t power the wheels. The only way the Volt can move along the road is using the electric motor that turns the front wheels.
The gas engine? It simply runs a generator that recharges the battery enough to keep the Volt moving for another 250-plus miles. Its function is not to move the car, but to extend the distance the battery pack can take it. Hence, the engine is a “range extender”.
In Volts that travel less than 40 miles a day and are plugged in daily, the engines may not get switched on for weeks, if ever. That’s why we call the Volt an “extended-range electric vehicle,” or EREV.
Delete parts; what’s left?
Or, we can put it another way: The Volt is an EV. Take out the gasoline engine, and it runs as an EV. But the Prius is gasoline-powered (with or without the ability to plug in). Take out the battery pack and electric motors, and it still runs as a gasoline car.
Yes, yes, we know “hybrid” means a combination of different power sources. But it’s important to understand the crucial difference between what we call hybrids today (e.g. Prius) and the plug-in electric cars of tomorrow, with or without range extenders.