Environmental concerns - energy - renewable energy.
1. Fossil fuels - alternatives.
Defined broadly, a biofuel is any fuel derived directly, or relatively quickly, from biomass such as wood, straw or other plant matter.
Contrasted with fossil fuels, such as oil and coal, which go through much slower geological processes to become usable fuels.
Examples of such broadly defined biofuels include wood that is burnt, and bioethanol, produced by fermenting plant matter.
Biofuels are generally regarded as renewable sources of energy, because of the relative speed with which the sources of biomass can be replenished.
However, the net effects on greenhouse gases and other pollutants varies, depending on the particular biofuel and the way it is used.
2. Fossil fuels - alternatives - liquid and gas fuels.
Defined more narrowly, biofuels can refer just to fuels in liquid and gaseous form.
For example, bioethanol used as vehicle fuel.