A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity.
They are highly efficient at producing energy and in most cases emit no pollution. If the hydrogen that fuels them is produced using clean, renewable energy like wind, then the entire chain is zero pollution. Hydrogen can also be produced from waste water and other waste materials, as well as being a leftover byproduct of manufacturing and thus available for use.
So, you can see why fuel cells are a Holy Grail of energy. If we can make them work on a mass scale, then oil consumption and carbon emissions would drop substantially.
More from DOE (where the image came from)