In general, hardwood comes from a deciduous tree which loses its leaves annually and softwood comes from a conifer, which usually remains evergreen. Hardwoods tend to be slower growing, and are therefore usually more dense. Softwood trees are known as a gymnosperm.
Hardwood is not a technical description or category; it’s more of a nickname that’s usually used to distinguish “hardwoods” from “softwoods.” Of course, the nicknames have some truth. Hardwoods as a group are harder and denser than most softwoods. But the real distinction lies in the type of tree—or more specifically—the type of seed that hardwoods come from.