The Janka test measures the density and hardness of a particular wood species. Although the Janka test is primarily used for hardwood species, it's often used to measure the density of bamboo floor types as well. The test is a good indicator of how much denting and wear and tear a particular floor can withstand. Generally, the higher a wood's rating on the Janka Hardness Chart, the more resistant it will be to most wear.
Additionally, the Janka test can determine how malleable the wood is in regard to nailing and sawing. While not always true, the hardwood species that are on the lower end of the Janka Hardness chart (e.g. Chestnut; North American Cherry) tend to be easier to work with for do-it-yourself installers.
So what does the Janka test consist of? The process of measuring the density of wood begins by embedding a steel ball that has a diameter of 11.28 millimeters (roughly 0.444 inches) halfway into the wood's surface. The force required to push the ball into the wood (measured in pounds-force, or lbf) indicates how dense and strong the wood is. For example, hickory hardwood has a Janka rating of 1820; this means that it took 1820 pounds of force to embed the steel ball into hickory's surface.
The hardness and density of wood is often determined by the direction of its grain. Measuring a wood's flat or horizontal grain (face) is the most general way to determine its hardness. Although vertical wood grain (edge) is tested, the results are not displayed on the Janka Hardness Scale. The results that are shown on the Janka Hardness Chart indicate the hardness of a wood's face, and not its edges (or "side hardness").