This page refers to the use of wood as a fuel.
Wood is an organic substance that has been used as a fuel for thousands of years. It can even be burned as biomass to produce electricity. Because of the water content in the fuel, energy density can change, but a standard value is around 16 Megajoules per kilogram. Even keeping warm by a fire is using wood as fuel.
|Energy Density||16.2 MJ/kg|
There is no question that wood is renewable energy, because it regrows once it has been cut. However, whether or not wood is a sustainable energy fuel is an interesting question, please see renewable vs. sustainable energy.