Wood is the primary content of trees. Everything in a tree, apart from the leafs, needles and bark is wood. Wood has been an important construction material since times immemorial, and remains in plentiful use today. In modern times, many of the traditional uses of wood have been taken over by metals and plastics.
Wood is often classified by hardness. The wood from most pine trees is rather soft, where the wood from oak is much harder. Additionally wood from different types of trees have different colors and grain densities. Because of these differences, and the fact that some woods take longer to grow than others, wood from different types of trees is valued differently.