Any vehicle is a hybrid when it combines two or more sources of power .In fact many people have probably owned a hybrids vehicle at some point. For example, a moped is type of hybrid because it combines the power or a gasoline engine with the pedal power of its raider. Hybrid vehicles are all around us. Most of the locomotives we see pulling trains are diesel -electric hybrids. Cities like Seattle have diesel electric buses these can draw power from overhead wires or run on diesel when they are away from wires. Giant mining trucks are often diesel electric hybrids. Submarines are also hybrid vehicles some are nuclear -electric and some are diesel electric. Any vehicle that combines two or more sources of that power that can directly or indirectly provides propulsion power is a hybrid. The gasoline electric hybrid car is just that a cross between gasolines powered car and an electric car. Hybrid electric vehicles HEVs are powered by two energy sources An energy conversion unit such as a combustion engine or fuel cell. And energy storage device such as batteries so ultras capacitors, fly wheels. A gasoline, methanol, compressed natural gas, hydrogen, or other alternative fuels may power the energy conversion unit. electric vehicles EVs. Because the HEV engine shares the workload with the electric motor, it can be constructed smaller. This reduction in size engenders weight reductions, leading to greater fuel economy. Also, HEV engines can be optimized to operate within a specific speed range characterized by better fuel economy and reduced emissions. This allows HEVs to eliminate the higher emissions and poor fuel economy associated with conventional ICE vehicles. Hybrid electric vehicles have the potential to be two to three times more fuel-efficient than conventional vehicles .In general hybrids capture energy lost during braking and return it to the on-board battery. This process is termed regenerative braking. HEVs generally dont ever need to be plugged in and charged - they recharge them selves during operation. Hybrid-concept vehicles are achieving fuel economy ratings of 80 miles per gallon. General Motors new Precept, for example, achieves a fuel efficiency rating of 108 miles per gallon. These are the most fuel-efficient and non-polluting vehicles the emissions that are released by these are almost zero thats why several countries prefer these to reduce the environmental pollution and the global warming which is the main problem of this generation. Emissions from the vehicles constitute the major part of the pollution so to reduce this several measures are being take HEVs are best suited for this. The hybrid electric vehicle operates the alternative power unit to supply the power required by the vehicle, to recharge the batteries, and to power accessories like the air conditioner and heater. Hybrid electric cars can exceed the limited 100-mile 160 km range-per-charge of most electric vehicles and have the potential to limit emissions to near zero. A hybrid can achieve the cruising range and performance advantages of conventional vehicles with the low-noise, low-exhaust emissions, and energy in dependence benefits of electric vehicles .In the coming years, hybrids can play a significant role in addressing several of the major problems faced by the world today climate change, air pollution.