We often speak of hard wood and soft wood forests. In reality, each type of wood has its own specific hardness. One way to measure that hardness is the Janka test. The test consists of pushing a ball 11.28 millimeters (0.444inches) in diameter half it’s diameter into the wood. The force required to do that is then measured.
The force can be recorded in a number of ways, including pound-force , kilogram-force, or newtons. The results can also either show side hardness, where the ball is pushed perpendicular to the grain (from the side of the tree), or end hardness, where the ball is pushed parallel to the grain (into the stump of the tree).
A good table of different side hardness Janka ratings can be found here.