For almost a century now we have been using the petrol fuelled internal combustion engine to power our cars and get us from point A to point B. A hundred years is a long time and in that time the engines have only become more powerful and efficient; so while it is safe to say that no one is denying its performance capabilities, it does not mean there are not issues.
The fuel used to run an engine needs to be capable of releasing a lot of energy, which means that it needs to contain a high energy density; but it also needs to be light enough to be carried within the vehicle and able to be stored safely at ambient temperatures. Petroleum is a rare thing in that it meets all of these criteria and does so very well, which is why its use is so prominent and also why it is so difficult to replace.
There are also environmental issues that arise from oil spills, which can occur during the extraction of petroleum from the ground or whilst it is in transit. A spill is especially damaging if it occurs at sea, as it spreads out across the water's surface, covering vast distances of sea, shoreline and beaches, affecting all forms of life that live there.
Environmental concerns aside there is the addition concern of petroleum, being a fossil fuel, having finite quantities; once its reservoirs have been depleted we will not be able to access any more. This alone is enough of a good reason to search for alternative fuels, or at the very least attempt to reduce the amount of petroleum we need to use.
One of the biggest steps we have made toward this end is the hybrid car, which combines the use of an internal combustion engine with an electric powered motor. The marriage between the two is ideal as sharing the workload between the two reduces the amount of fuel that needs to be used, and as neither need to be as big as they would if acting individually, the vehicle's weight is reduced; along with the energy required to move it.
The Prius is capable of acting like what is known as a series hybrid, and also as a parallel hybrid. A parallel hybrid is capable of using both the engine and the motor at the same time or independently to affect the car's motion; whilst in a series one the engine can be used in tandem with the motor to perform background tasks, such as charging the battery or powering the wheels, without affecting the car's speed.
There is another form of hybrid called a mild hybrid, which is not capable of ever operating on electricity alone, and the electrical motor is only ever used to assist the engine when extra energy is required; however when the car is still the engine shuts down, and is started again by the electrical motor when the car is put in gear.