Hybrid vehicles are automobiles that combine two or more different power sources to propel the vehicle. The most common type of hybrid vehicle is a gasoline-electric hybrid, which uses both an internal combustion engine and an electric motor powered by a rechargeable battery pack. Other types of hybrids include plug-in hybrids that can be charged from an external source, and vehicles with alternative powertrains such as hydrogen fuel cell or biodiesel engines. The goal of these vehicles is to improve fuel efficiency and reduce emissions by using the most efficient power source for different driving conditions.