Wood is a material found as the primary content of the stems of 'woody plants', especially trees, but also all shrubs. These perennial plants are characterised by stems that grow outward year after year, and that are composed of cellulose and lignin based tissue. Plants that do not produce wood are called 'herbaceous'; this group of plants includes all annual plants, many perennial plants, and most submerged and floating aquatic plants.
The woody tissue is formed by the plant for structural purposes, and because it is an effective and efficient structural material, it is useful to humans. Wood is made of cellulose fibers, held together with lignin.
When cut down and dried, wood is used for many different purposes. Wood that is broken down into fibers is called pulp, which may then be made into paper. Artists and craftsmen shape and join pieces of wood with special tools, which is called woodworking or carpentry. Wood has been an important construction material since humans began building shelters, and remains in plentiful use today.
Wood is commonly classified as either hardwood or softwood. The wood from conifers (e.g., pine) is called softwood, and the wood from broad-leaved trees (e.g., oak) is called hardwood. This classification is sometimes misleading, as some hardwoods (e.g., balsa) are actually softer than most softwoods.
Additionally, woods 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 kinds of trees have different qualities and values. For example, while mahogany is a dark, dense hardwood which is excellent for fine furniture crafting, balsa is light, soft, and almost spongelike, making it useful for model building.