Biomass energy is energy that is derived from what were once living organisms. That might mean plant matter or the bodies of crustaceans or other animals.
Wood is one example of biomass energy, and wood has long been burned to release heat. Peat, which is made from vegetable material that has partly decomposed, is another example of biomass that is found in acidic soil.
Though it is abundant and very varied in the forms that it takes, biomass energy can take a long time to renew. Peat deposits can take hundreds of thousands - or even millions - of years to lay down, for example, as is evidenced by the wooly mammoths that have so famously been unearthed from peat bogs. Forests can be grown sustainably and their trees used for firewood, but this requires careful management.
In sum, biomass energy can be described as sustainable if it is well managed, and as renewable if used carefully.
Laura