When talking to anyone involved with timber, you may often hear the terms hardwood and softwood. These terms can be a bit confusing because they actually have nothing to do with the density of the wood. The softest wood ever measured is Balsa and this wood belongs to the hardwood category. Hardwoods are typically harder than softwoods, but not always.
The terms hardwood and softwood actually have to do with reproduction and the seed, specifically if the seed is encased or not.
Hardwoods reproduce by seed (like softwoods) but hardwoods produce flowers and their seeds have some type of cover. Hardwoods belong to the group angiosperms and diverged from the softwood group +200 million years ago.
Softwoods also reproduce by seed but do not flower and have an exposed seed. They belong to the group gymnosperms.
I find the easiest way to determine if a wood is hardwood or softwood is that hardwoods typically lose their leaves in winter (deciduous) while softwoods typically are evergreen.