A solar fuel is a synthetic chemical fuel produced from solar energy.
Solar fuels can be produced through photochemical (i.e. activation of certain chemical reactions by photons), photobiological (i.e., artificial photosynthesis),  and electrochemical reactions (i.e. using the electricity from solar panels to drive a chemical reaction). 
Solar fuels can also be produced by thermochemical reactions (i.e., through the use of solar heat supplied by concentrated solar thermal energy to drive a chemical reaction).
Light is used as an energy source, with solar energy being transduced to chemical energy, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.
A solar fuel can be produced and stored for later use, when sunlight is not available, making it an alternative to fossil fuels and batteries. Examples of such fuels are hydrogen, ammonia, and hydrazine. 
Diverse photocatalysts are being developed to carry these reactions in a sustainable, environmentally friendly way.