Hardwood is wood such as Oak, Maple, Beech and Mahogany, just to mention a few. They are very strong and hard and are usually broadleaved tree as distinguished from that of conifers. The term may also be used for the trees from which the wood is derived; these are usually broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwoods are in general harder than softwood. However, some hardwoods, such as basswood, are comparatively soft, while some softwoods, such as yew, are comparatively hard.
Hardwood is not necessarily a harder material (more dense) and a softwood is not necessarily a softer material (less dense). 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. In conifers like pines, these seeds are released into the wind once they mature. This spreads the plant’s seed over a wider area.
For the most part, angiosperm trees lose their leaves during cold weather while gymnosperm trees keep their leaves all year round. So, it’s also accurate to say evergreens are softwoods and deciduous trees are hardwoods.
The hardwood/softwood terminology does make some sense. Evergreens do tend to be less dense than deciduous trees, and therefore easier to cut, while most hardwoods tend to be more dense, and therefore sturdier. But, as the classification of balsa wood demonstrates, there is no minimum weight requirement to become a hardwood.