You know that hissing sound your board makes when you're going fast? Those are your bearings at work, keeping your wheels turning and ensuring that you have a smooth ride.
Skateboard bearings are inserted inside the wheel wells and allow your wheels to spin when they're attached to the truck's axle. As such, good bearings are essential to a street or vert skater. If one of your bearings gets rusty or outline, the wheel it's attached to can spin differently than the others in your set and your face may become acquainted with asphalt.
Pave the Earth...
A skateboard is more than just a bunch of wheels nailed onto a plank. A good skateboard has lots of little components, and skaters will soon cultivate interest in brands that fit their style of riding. Skaters, like many athletes, tend to fetishize their favorite brands of equipment, so you can get stickers, clothing, and other merchandise.
If you're putting together your first skateboard, rest assured that putting in bearings is easy. While you do have to be sure that you're using proper skateboard bearings that are correctly sized to your wheels, all you have to do is push them in with your thumb and pop your wheels on your trucks. Be sure not to run your board through any water or ride on rainy days, as you could ruin your bearing. Beyond that, it's usually a good idea to go for quality over low pricing. Cheap bearing aren't too good at surviving the rough treatment skaters throw at their decks and it doesn't take much to knock them out of shape. For what it's worth, I used to ride Spitfires and they never did me wrong.