A hydrogen car is one that uses hydrogen as a fuel rather than the more conventional petrol or diesel.
To confuse matters, though, there are two very different ways for a car to run on that hydrogen.
In the first, hydrogen is burnt in a conventional internal combustion engine, just as petrol and diesel normally are.
In the second, a chemical reaction between hydrogen and oxygen in a fuel cell produces energy that powers an electric motor to drive the car.
In both cases, the advantage is that there are no CO2 emissions. Instead, the only emissions are water and heat.
That all sounds great and it is but the trouble is that there are also plenty of problems with using hydrogen as a fuel, not least of which is how to produce the stuff in the first place.
To cut a long story, its not easy, uses energy and can produce CO2 emissions which rather defeats the object. Oh, and it also costs a fair bit of money.