Wood is the fibrous tissue found in many trees. It is used as a fuel and as a (organic) construction material
 For burning
Wood can be used as is for heat, cooking, ... Because of the scarcity of wood in many places, other biomass can be used as feedstock, but may have to be densified first. For this task pelletizers are commercially available.
 For construction
Wood can also be used for construction. The species of wood, together with the location in the tree from where the wood is obtained chosen greatly affects the durability.
The durability of the tree species is categorised in classes (class 1 being most durable, class 5 being least durable). The terms "hardwood" and "softwood" refer to these classes; softwoods being those in the lower classes.
In addition, we need to distinguish between wood obtained from the center/core from the tree (heartwood) and wood obtained from the outer area (sapwood). Heartwood is generally much stronger/more durable than sapwood.
Finally, wood can also be heat treated and treated with oil (tung oil, linseed oil, ...) in order to make it more durable. This is generally done with wood from tree species from the lower durability classes, and/or with sapwood. This is because wood from the higher durability classes/heartwood does not let itself be treated as easily. By using treated wood from native tree species (e.g. oak if you live in Europe, ...), you can reduce the ecologic footprint quite a bit. This as trees are grown especially for the industry (protected with FSC labels) rather than being taken from woodlands/illegally harvested without replanting. In addition, tree species from the lower durability classes also grow much more quick (some species of trees can be fullgrown in tens of years -sometimes even quicker rather than hundreds of years).