Hydrogen is one of many potential replacements for gasoline as fuel for ground vehicles. It is not a power source in itself, but an energy carrier that is produced rather than found naturally. It tends to be made from methane or other fossil fuels, but it can also be obtained from sources like wind, solar or nuclear power. These are readily available non-fossil energy sources that themselves are unsuited to propelling vehicles, but if used to make hydrogen would allow for a vehicle economy entirely void of carbon dioxide emissions. As farfetched as it might seem, it is very much a possibility if one takes into account that vehicles such as airplanes, submarines and space shuttles already run by hydrogen at least partially.
Hydrogen motor vehicles can either be electric cars or of the combustion engine variety. The former type would be powered by a fuel cell that lets hydrogen react with oxygen to run an electric motor. However today's hydrogen fuel cells tend to be highly fragile and would currently not endure the bumps and vibrations that motor vehicles inevitably are subject to. As they moreover require rare and expensive substances like platinum as catalysts they would also be costly. Less expensive alternatives are currently under development, but if cheaper models would break just as easily it might be a while before hydrogen fuel cell cars become commonplace. The structure of the fuel cell would also cause problems, being that it contains an internal vaporous water supply that could solidify in temperatures below freezing. As the fuel cell process generates heat as a by-product it would not be an issue in a running engine, but if left turned off until frozen solid it would not be able to start and could, given the aforementioned fragility, perhaps even be permanently damaged.
A cold weather option could instead be vehicles with combustion engines converted to burn hydrogen instead of gasoline. Such engines have already been built, and they are cheaper by far because of how they are based on existing combustion engine technology. However they would not currently be good for much in commercial vehicles given the general lack of a hydrogen infrastructure for refuelling. There are currently a number of experimental Hydrogen refuelling stations operational in the US, but they are few and only exist in certain parts of the country. A nationwide expansion would be necessary for hydrogen to ever become a large-scale gasoline replacement, and for such a project to become reality investments of millions of dollars would be required.
If these factors alone did not speak against hydrogen as vehicle fuel in the near future, the currently inefficient methods of producing and storing hydrogen would be problematic as well. Granted, many companies are working on technologies that could enable electrolysis of water on a large enough scale and for a cheap enough price for hydrogen to become a feasible fuel source for vehicles, but as with the problems with the fuel cell, the completion thereof probably cannot be expected to occur in the near future.
That does not mean that we will have to wait to see hydrogen vehicles in use. Hydrogen buses are currently on trial in many locations of the world, a fleet of hydrogen-run London taxis is planned to be ready for trial for the London 2012 Olympic Games and many car producers have already produced functional hydrogen demonstration models of personal vehicles. Hydrogen is, in other words, indeed a potent alternative to gasoline, but in all likelihood the technology required to put it to use as such needs years to mature before it is ready to be launched for the public.