Bearings are comprised of a large outer ring and a smaller inner ring. Between these rings are small steel balls. These are what enable the wheels to turn. On either side of the rings are covers, called shields, which prevent the bearings from getting dirty. In order to keep the balls in position, they are contained inside of a so-called “ball cage.”
You can also choose to skate with ball bearings without shields, which are open or semi-open. If you do skate with open or semi-open bearings, they will indeed be more susceptible to contamination, but they will be somewhat faster. Many skaters enjoy the slightly louder sound of a skateboard with open bearings. - States SkateDeluxe
A strong divide in the skate community, but rarely discussed, shields or no shields? 
Either way you go, you are still skating, and that is all I care about. But... there have been heated debates between firm believers on both sides of the argument on which is better. With shields, they stay cleaner and protected longer, the grease stays in and they stay faster longer... so they say... With-out shields bearings have less resistance, roll faster and the steel ball bearing will kick out any dirt or debris allowing the bearings to self clean whenever they are spun at high speeds... so they say... In my humble opinion, it doesn't truly matter. bearings are going to wear out, get dirty, and need replacements, it's all about preference. I have ridden my bearings both ways, I personally like the sound it makes with out shields.
Which ever you prefer is the best way to go for you. Just like everything else in skateboarding, enjoy your own personal preference!
