The definition of biomass is pretty loose, but generally means any energy you can get from things that used to be alive. This can be anything from rotting food and garbage, to vegetable oils, to wood and other fibrous plants like rice husks. If you look at it from a big picture, people have been burning wood (for example, trees) to make heat for a long time, likely since fire was first harnessed. Although this is fairly simple, nowadays we can produce liquid fuels (such as biodiesel or ethanol) and gases from plant material and garbage. As you might guess, biodiesel and other liquid fuels can then transported and burned in engines to do useful work.

As I mentioned, the term "biomass" is very general, and biomass can be in the form of rotting food, decomposing garbage, rice husks, wood, vegetable oils, or even cow poop! Because of this, there is no molecular formula or molecular structure.

It can be almost anything. The most common biomass on Earth that we deal with is wood, which is composed of cellulose and lignin. Cellulose is just a polymer of glucose, a simple sugar. Lignin is a bit more complicated, and includes tarry substances as well as more cellulose.