Wood is a byproduct of trees. It is mostly made up of water, cellulose and lignin. Water and cellulose are the soft squidgy bits, whilst lignin is the hard bit. If you hit a tree whilst in possession of a reasonable amount of kinetic energy, it is the lignin that causes the injury, not the water or the cellulose.
It also used as a term to refer to a buch of trees, much like a forest. EG. "The Woods". According to The Bard, Birnam Wood came to Dunsinane.