A Hydrogen car is an automobile which uses hydrogen, generally obtained from water using electrolysis, as its primary source of power for locomotion. (This has absolutely nothing to do with fusion of hydrogen.)
The most straightforward way to design a hydrogen car uses an electric motor powered by one or more fuel cells. The primary technical obstacle is the storage of hydrogen, which would need either a cryogenic tank to store the fuel in liquid form, or a pressurised tank to store it as a gas.
The main benefit of using pure hydrogen as a power source is that it uses oxygen from the air to produce only water as exhaust.
An existing conventional car cannot easly be converted to run from hydrogen, due to a number of inherent differences in the technologies. Other renewable energy sources, like biodiesel, are more practical for existing automobile conversions.
See also: Future of the car