Balsa is the lightest usable wood in existence. It is light enough to be used for various purposes, and it is strong enough to support some weight.
Balsa is light for two reasons; it has large spaces between its cells and a small amount of lignin. The amount of lignin, a material that holds the cells and fibers of plants together, is minimal in balsa, meaning that only about half of the wood is solid. When alive, the empty spaces allow the balsa tree (Ochroma pyramidale) to draw water from the ground.
While balsa is not the lightest wood in existence, it is the most usable type as there is enough lignin to provide the wood structure. All other woods softer than balsa lack a sufficient amount of lignin to support weight, causing them to be unusable.