skateboard wheels are for the most part made with polyurethane. 51 mm wheels are slower riding. Good for street, park, and pool skating. Good for smaller skaters. Bigger the wheel the faster it is. These wheels look like they would be standard issue and preference of most "street" skaters.
For the Record, Those wheels retail for about 25 bucks, My boyfriend Owns the oldest privately owned skate park in the world so, I would know :) Have a great day!