What is fuel used for?

Answer 1

Most fuels are derived from oil. A process called cracking helps take oil and split out all the various chemicals (into fuels, light oils, heavy oils etc). For more info see: http://en.wikipedia.org/wiki/Hydrocracking

Answer 2

It's burnt either in the cylinders of internal combustion engines which provide energy when they are running, or the heat given off when they are burnt in other ways is used, for example, to heat water to produce steam which powers steam turbines and steam engines (not so much the latter these days)

Answer 3

Fuel is used for burning for heating, cooking, driving engines and generating electricity.