Janka Hardness Rating
The relative hardness of wood species is measured using the Janka Hardness Rating. This test measures the force needed to embed a steel ball (.444 inch in diameter) to half its diameter into the piece of wood being tested. The higher the number, the harder the wood. This means the wood is more resistant to indentations.
Different species also have varying degrees of hardness. Hardwood floors are a natural product and are susceptible to dents (not covered under manufacturer warranties unless specifically stated). If you expect your floors will take a lot of abuse, consider a species at least as hard as red oak based on the hardness chart below.