Answer:
Hardwoods are denser, hence harder. They come from deciduous trees that lose their leaves in the winter: aspen, poplar, birch, maple, etc.
Softwoods are less dense. They come from coniferous (evergreen) trees that lose and replace their needles continuously: pine, spruce, cedar, fir, etc.
Oak, ash, elm, maple and mahogany are all hardwoods.