Probably the most important thing about your longboard ride is the wheels. But knowing which longboard wheels you want or need takes some experience to figure out. It all depends on the way you ride and the terrain you're on. If one person rode around in a smooth parking lot or skate park, they wouldn't need the same wheels as someone riding around the streets and the variety of pavements, cements, and asphalts you can hit in just a short distance in your neighborhood. Of course, you'll figure out your own longboard wheel preferences, but for now, here's a place to start.
Though most all longboard wheels are good on smooth pavement, you'll want softer, thicker wheels for rough ground rides over bumps and gravel. Soft wheels offer a smoother ride and a better grip. Soft wheels are good for cruising on your longboard and not so much for doing tricks, though you really won't be doing tricks on your longboard, anyway. And also remember that the bigger the wheel, the bigger the turn radius. Basically, this means that while you will be able to cruise easily, that's pretty much all you can do. Safely, anyway. And if you're a beginner, be careful of that first easy cruise … stopping can be really hard until you get some practice, so watch your speed.
Medium-grade wheels are purchased by the average skater and the beginning longboarder. Think of these as an ''in-between'' wheel. This means that these wheels are used by riders who use their longboard for a variety of reasons, from vert or speed skating to cruising.
Hard wheels are used by street skaters and vert skaters, mainly. They're more sturdy and stable for trick skating.
Large longboard wheels are for riding faster, so large wheels are favored by longboard riders that spend quite a bit of time hitting the ramp. If you are riding on an incline with your longboard, you will probably want harder wheels for more glide than grip. Since most longboards are typically used for cruising and not tricks, larger longboard wheels will accommodate them better.
Much like medium-grade hardness, you may also want a medium-sized wheel for your longboard if you tend to do a mixture of tricks and cruising. You might combine medium hardness and medium-sized wheels so that you have a good balance of agility and speed.
Smaller wheels are usually used for trick skating because it's easier to land tricks when your board is closer to the ground. This gives the board excellent maneuverability in order to land more technical tricks.
All that being said, if you're a street skater, you'll probably want smaller, harder wheels. If you're mainly using your longboard to cruise, then large, soft wheels are probably your best choice. And if you use your longboard for a mix of street skating and cruising, wheels right in the middle will be just right. Choosing the correct durometer and size of your longboard wheels will make for a smoother, faster, and all around better ride.