The basic premise of a hybrid car is to combine the most efficient power sources for the action the car is taking at a given time. This should maximize fuel savings and minimize carbon emissions. Put simply, hybrid cars run on a combination of electricity and gas. For example, the Toyota Prius -- the most popular hybrid -- has a transmission capable of splitting power between the electric motor/generator and the gasoline engine (hybrid gas engines typically are smaller than those in conventional cars). The electric motor powers the car at low speeds, up to about 15 miles per hour (24.1 kilometers per hour). When the car picks up speed, the gas engine kicks in. What's more, as the gas engine cruises along, it powers up the electric batteries. If the car has to accelerate more, both engines can work together to give it plenty of boost. If the driver applies the brakes or slows down, the hybrid car stops using power and instead harnesses power from the car's spinning wheels and stores it in the generator. The Prius can achieve higher fuel efficiency in the city than on the highway because of the use of the electric generator [source: Auto MD].

The careful orchestration of these engines is one reason why hybrids are fuel efficient and good for the environment. The Chevy Volt, for instance, uses on-board computing to run the engines and help the driver maximize efficiency. There has been some debate about whether the Volt really is a hybrid or an electric vehicle in more recent years, however. General Motors began to market the car as all-electric and some critics said it still was a hybrid because it had a gasoline engine to charge its battery. Unlike the third generation (2010) Prius design, the 2012 Volt was designed to use electrical power all of the time, at least until the generator runs out. At that point, which is reported to be about 25 to 40 miles (40 to 80 kilometers), the gas engine kicks in [source: Bond].

Hybrid designs continue to change and advance, so it's hard to say what the future holds, much less what to expect in new models from one year to the next. Manufacturers around the world continue to look for new answers to improving battery range and lowering emissions.