A biofuel is stuff made from plants or animal waste that you can burn, or otherwise use, to get heat or power. It can be a liquid, like plant alcohol for cars, a gas from rotting plant matter, or even solid stuff like wood pellets. So it is the umbrella word: bioethanol and biogas are kinds of biofuel, and bioenergy is often the power you get when you use them. People talk about biofuel when they want energy that did not come from old oil under the ground. Though actually, “from plants” does not automatically mean “kind to the earth” — fields, water, and smoke still matter, and some biofuels are much cleaner in practice than others. You will see the word in news and school texts about energy. It is more formal than “plant fuel,” which nobody really says.