A substance that produces useful energy when it undergoes a chemical or nuclear reaction. Fuel such as coal, wood, oil, or gas provides energy when burned. Compounds in the body such as glucose are broken down into simpler compounds to provide energy for metabolic processes. Some radioactive substances, such as plutonium and tritium, provide energy by undergoing nuclear fission or fusion.
Alternative fuels tend to be made up of small, fairly simple molecules; for example, here are schematic chemical diagrams (C denotes a carbon atom, H is hydrogen, and O is oxygen) of methane (CH4), propane (C3H8),
Petroleum fuels are blends of lots of different chemical species; in general, the molecules of a liquid petroleum fuel are pretty big and complex. Here is isooctane (C8H18), typical of the molecules found in gasoline (I had to spread out the structure a bit to get all the hydrogen atoms to fit in the picture--all of these molecules are, of course, three-dimensional, but some squish into a plane better than others!), and
this monster is cetane, or n-hexadecane (C16H34), typical of diesel fuel.