Skateboard wheels are made out of polyurethane. Many say they are made from urethane which is a commonly mistaken as it is a different material.
Polyurethane is a grippy and resilient material used for all sorts of things from rigid foam and car tires to carpeting. It has been an excellent material for skateboard wheels since the seventies.
There are two main types of wheels for skateboarding. Small, hard street wheels and big, soft cruising wheels.The small ones are round in profile and the big ones are more square shaped.
There are lots of options inbetween though. For pretty much any style you want. Stick with brand name and logo wheels. Cheap skateboard wheels will wear quickly and flatspot easily.
Most wheels are 50-60mm in diameter and 30-50mm wide. They all have a hole through the middle for the axle with a space on either side to hold the bearings.
Most are shaped so that they cover and protect the axle nut. This keeps it from getting to beat up from skateboarding.
Modern wheel shapes:
Street skateboard wheels are rounded and symetrical to use less material and keep the wheel light for flip tricks. The shape has less contact with the ground making it more responsive. It also exposes a lot of the hanger for lots of control during grinds. The rounded edge makes it easier to slide along the ground and into tricks. There is just less when to grip and it lets you pull long nose and tail slides.
Oldschool wheel shapes:
Square and asymmetrical. Protects the axle nut and gives the most surface area in contact with the ground for a smoother ride. Better grip and going fast.
The hardness of the wheels affects the way they ride. It is rated on a durometer scale. A soft wheel is around 87A and a hard wheel is 98A. For comparison a rubber band would be 25A and car tires are around 70A.
A soft skateboard wheel will be grippy, quiet rolling and smooth over rough terrain. They are good for going fast or filming. Soft wheels often coming in larger sizes for speed.
Your typical street wheel will be hard. This allows for better sliding in and out of tricks. They are louder and rougher rolling but lighter and more responsive.
There are also wheels with dual durometers. The outer wheel is a softer material and the inner core is a harder urethane. This gives the best of both worlds. It often comes at a higher price but is often worth the added expense.
You can get almost any colour you want. I always liked white wheels. Even though they often fade to a yellowish colour I still like the look of white wheels on my board. I can see the light blur that they make when spinning in a flip trick. They look like they are glowing to me.
Be sure to rotate your wheels once in a while. They will wear differently. I find that my back wheels will wear much faster than my front wheels. The outside of the wheel will also wear faster than the inside. This will cause the wheels to turn into a coned shape.
I switch the front and back wheels around every few weeks and always flip the outsides to the inside to keep everything even. Just loosen the axle nut and remove the two small washers on either side of the wheel. Try not to lose the spacers. Put them back on with one on either side of the wheel.
A good set of wheels can make the street or bowl feel like ice. After a few months on an older pair of wheels. A new set can really make a difference in how good the skateboard rolls and feels. Rotating them will make them last longer but nothing is as good as a new set!