You can also view previously answered questions in the HSW Question Archive . For example, balsa wood is one of the lightest, least dense woods there is, and it's considered a hardwood. The distinction between hardwood and softwood actually has to do with plant reproduction.
All trees reproduce by producing seeds, but the seed structure varies.
Hardwood trees are angiosperms , plants that produce seeds with some sort of covering. This might be a fruit, such as an apple, or a hard shell, such as an acorn. Softwoods, on the other hand, are gymnosperms . These plants let seeds fall to the ground as is, with no covering. Pine trees, which grow seeds in hard cones, fall into this category.