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 practical technical solution would be to use an electric motor powered by fuel cell. 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 gas.
The main benefit of using pure hydrogen as power source is that it uses oxygen from the air to produce only water as exhaust.
To use hydrogen, you change from a petrol to an hydrogen car, this is, buy a new car. With another 100 % pure renewable energies, like biodiesel, you can use your nowadays petrol car.
See also: Future of the car
wikipedia.org dumped 2003-03-17 with terodump