Skateboard wheels are made up of polyurethane and are shaped to contain bearings. Three main variables should be considered when selecting wheels. The first is size (diameter), the second is durometer (softness). Generally speaking, wheel sizing options increase by one milimeter increments with 50 mm considered small and 60 mm considered big. Lower durometer valued wheels (starting at about 70) are softer while higher durometer valued wheels (ending at about 100) are harder. Smaller, harder wheels are meant for performing aerial skateboard tricks under smooth conditions (ex. a skateboard park with fresh concrete). Larger, softer wheels are tougher to get off the ground (not ideal for tricks) but are great for tackling non-ideal conditions (ex. old asphalt).
The last variable to consider is the shape of the wheel. Although all skateboard wheels will (obviously) be round, some are cut to specific shapes with different intentions in mind (for ex. certain wheels are shaped to facilitate smoother grinding). This variable is the least important, in comparison to the first two.