The difference between trees and their hardness, thus defining their use, comes down to their cellular and structural level.  For all you science nerds out there,  the following is derived from A Guide to Pest Control in Australia (PW.Hadlinton).  While the book focuses on local pests and their management, it has a great section on dealing with pest associated with tree damage such as termites and borers.  As such it provides great details on the trees themsellves.

“The conductive cells in pines are elongate and needle-like, vary in length from 3-15mm and are known as tracheids. The walls of these cells may be modified by thickening of cellulose from within.  Certain areas in these cells, the pits, are not thickened, and conduction from cell to cell occurs  through these.

When viewed with an X10 hand lens the tracheids appear close together, evenly distributed and in zones of dark and light colour. The pale zone consists of thin-walled cells (early wood) and the darker zone of thick-walled cells (late wood). One dark and one light zone usually represents a growth ring.

Wood parenchyma cells are split off from the cambium and are referred to as ray cells, which are associated with food storage. Resin, a gummy aromatic material associated with resistance to insect attack, is produced in the parenchymatous tissue.

Hardwoods have a small fibre length, thus producing a pulp inferior to that of softwoods. Thestrengthening tissue of hardwoods consists of fibres with pointed ends and few pits. The storageissue is abundant, being mainly parenchyma cells and rays.

The conducting region for mineral salts in hardwoods is the sapwood which contains vessels or pores (open-ended pipes or tubes) and arranged in a characteristic pattern. The many patterns ofvessels, fibres, parenchyma and ray cells assist in the identification of the timber, even to the species.”