Technology Review has created a useful animated primer on regular hybrid cars, plug-in hybrids and 'extended-range' electric car. The latter is what the Chevy Volt will be; A plug-in hybrid, but unlike with regular hybrids and plug-in hybrids, the gasoline engine cannot turn the wheels directly. All it can do is charge the battery, which then powers electric motors that turn the wheels.
TR's animation shows operation of the vehicles both in charged conditions and when the battery is depleted. It's well-done and should answer the questions of those who are not familiar with the differences between these types of vehicles that are bound to become increasingly popular in the near future. You can see the animated primer here: ::Electric Cars Primer