Extended-range electric vehicles are similar to a pure electric or plug-in hybrid vehicle in that they have a plug-in battery pack and electric motor to drive the wheels.
So you can charge them easily from a socket at home or at work and the range will be sufficient for most journeys.
But if you need to travel farther and battery is depleted, an internal combustion engine kicks in and acts as a generator to keep the battery at minimum level until you can next charge it.
So unlike plug-in hybrids, the electric motor always drives the wheels.
Range extenders can have a pure electric range of up to 125 miles. This typically results in tailpipe emissions of less than 20g/km CO2.