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What are they?

Fuel cells are devices which convert chemical energy directly to electrical energy. This is very different from conventional combustion based power plant which convert chemical energy to thermal energy, then thermal energy to kinetic energy, and only then kinetic energy to electrical energy.

The thermal to kinetic and kinetic to electrical conversion stages have efficiency losses associated with them which engineers have spent the last 150 years or so trying to reduce. The key loss however is in the combustion process (chemical to thermal stage). Due to the inherent thermodynamics of combustion there is an ultimate efficiency which cannot be exceeded by any combustion engine - The Carnot Limit - a limit which does not apply to fuel cells.

Using hydrogen as a fuel (which can be extacted from hydrocarbon fuels or renewable sources) a fuel cell electro-chemically oxidises the hydrogen using oxygen from the air generating electricity and some heat.

The fuel cell makes more efficient use of the fuel and produces fewer pollutants e.g. reduced nitrgen oxides and carbon dioxide emissions, and no particulates.