The relative hardness of wood species is measured using what is called a Janka Hardness Rating. This test measures the force needed to embed a steel ball (.444 inch in diameter) to half its diameter in the piece of wood being tested, with the rating measured in pounds of force per square inch. So with this rating system, the higher the number the harder the wood.
Wood hardness is important since one of the key considerations in selecting the species of wood floor your floor should be how much resistance the wood has to scratches and indentations. For example, if you have a dog with long nails then scratching the floor is a consideration and you should select a species with a higher rating such as hickory, maple, oak or ash.