Plug-in Hybrids work in the same way as normal hybrids – there is still an electric motor and an engine, but usually with PHEVs, the electric motor is more powerful, so it is able to drive the car up to 70mph or so on pure electric power. It also has a bigger battery, so that it can drive up to 30 miles or more on a full charge.
Because the battery is so much bigger, it needs to be charged from the mains electric supply, rather than just through braking force. A full charge typically costs around £1 or so, which gives 25-35 miles of range. The average driver in the UK drives just 22 miles a day, so it would be quite possible to drive day-to-day without the engine using any fuel at all. Of course, if you need to make a longer journey, then once the battery is depleted, the car switches to being a hybrid car. So you can drive as far as you want, although you will need to stop to put some petrol in it eventually!
PHEVs work really well for people who do a short regular commute, but occasionally need to do longer journeys. In order to get the best economy from the car, you would need to charge the car overnight every day to keep the battery topped up – so a driveway or garage would be recommended.
A hybrid car is one which is powered by two forms of propulsion – normally, this is a combustion engine, together with an electric motor.