The term "biomass" describes biological material that is derived from living or recently living things. There are many different uses for biomass, but most of the focus on biomass follows one of two paths: bioproducts and bioenergy. In a very general sense, it could be minimally processed and used as mulch or as animal bedding and most of the time we speak about biomass we are referring to the plant based side of it.
Bioproducts include a whole collection of things that can be derived from processed biomass. Most of the products made from petroleum can be made from biomass, including plastics, certain cosmetics, glues, antifreeze, and an enormous number of chemicals essential to modern life.
Bioenergy is energy derived from biomass. The biomass can be burned; allowed to decompose to where it releases methane and other gases that can be burned; or it can be digested and the products, including combustible gases, collected. Burning it releases heat, which can be used simply to heat a space, or it can be used to produce steam which can power turbines to produce energy. Among the useful bioenergy products, the most common are ethanol and biodiesel.
Converting biomass to bioenergy has two significant benefits over using fossil fuels:
Fossil fuels, by contrast, are not renewable--we have to continue to tap a finite resource each time we need more--and using them puts carbon into the atmosphere that was otherwise sequestered away deep in the earth. Of course, fossil fuels derive from biomass, as well, but it is renewable only over many thousands of years.
For more information on how biomass is converted to useful products, The U.S. Office of Energy Efficiency and Renewable Energy has a good primer on the subject at their site.
Last updated April 1, 2019