The Janka hardness test is a common test that determines how much force is required to embed a steel ball into a plank of wood half the ball's diameter. The .444 inch ball is pushed into the wood and this determines the woods hardness. There are some other factors that the test does not consider, such as grain direction, thickness and when evaluating flooring, the underlayer. However, as a rule of thumb, its a good determination used by wood manufacturers of the suitability of the wood for various purposes.
The hardest wood in the world ia Australian Buloke, the softest is South American Cuipo which is over 200X as soft as Buloke.