Hybrid vehicles combine power from a gasoline engine and a battery-powered electric motor. The batteries that power the electric motor, or motors, are charged by the gasoline engine when it's running, and also charged a little bit by energy captured while braking or coasting. So unlike an electric car, current hybrid models don't need to be plugged into an electric outlet. (Though that option is coming soon. More on that below.)
Electric motors, it turns out, make good partners for gas engines. They're especially good at generating something called torque. Torque is the actual twisting force that moves a car - or anything else for that matter. Torque is most necessary when you start off from a stop, and need to "get the car going." And if lots of your torque comes from your electric motor, your gasoline engine can be smaller. Voila! Better mileage!
The Civic Hybrid's electric motor, for example, produces a measly 23 horsepower, but 76 lb.ft. of torque - a good deal of torque, for such a small motor. Most gasoline engines are as big as they are so they can produce plenty of torque for acceleration, and need only a fraction of their available power while cruising. In a hybrid, however, because less torque is needed, the gas engine can be smaller. It just needs to be big enough to keep the car at cruising speed on the highway.
To further save fuel, most hybrids also incorporate an "engine shut-off at idle" feature. The gasoline engine automatically stops running when you stop at a light, and then instantly restarts either when you touch the gas pedal - or when you take your foot off the brake pedal, as happens with the Honda Civic Hybrid. The result is an increase in fuel economy of one to two MPGs around town for smaller cars. More if you're driving a car with a huge V8 engine. Along with that increase in MPG, comes a commensurate decrease in pollution. "Engine shut-off at idle" will eventually be on all cars, as well it should, because it also makes it a lot easier to breathe when you're walking through a busy intersection!
The plug-in hybrid adds a twist to this formula. A lot of people are walking around with the mistaken impression that regular hybrids need to be plugged in--nope. But just to confuse you further, the plug-in hybrid (examples include the Chevrolet Volt, the Toyota Prius Plug-In Hybrid, the Ford C-MAX Energi, the Fisker Karma) has a bigger battery pack and it does plug in for 10 to 50 miles of all-electric driving.
The basic idea is you get the best of two worlds--there's all the advantages of a regular hybrid, including great fuel economy and stellar range, plus the opportunity to travel fully electric. In fact, some owners of these cars are saying that they rarely use the gas engine at all. Expect to see many more plug-in hybrids on the market, because the format is proving popular.
One of the biggest problems with plug-in hybrids is giving them fuel-economy ratings that normal people like us can understand. The Volt, for instance, is rated by the EPA at the equivalent of 93 miles per gallon when it's in electric mode, and just 37 mpg with the gas engine running. Confusing, but it shows why owners have an incentive to run solely on that 16-kilowatt-hour battery pack.