What is a ball bearing, anyway?
Ball bearings are formed with an outer ring, an inner ring, a cage or a retainer inside, and a rolling element inside, typically a ball (which is why they are called ball bearings). Roller bearings are formed using a roller instead of a ball, which is why they are called roller bearings (Yes, finally something that makes sense!). Other bearings look just like metal tubes, called plain bearings or bush bearings. They look like sawed off pipe or tube.
The principle of bearings is the same principle behind the wheel: things move better by rolling than by sliding. They are called "bearings" because they bear the weight of the object, such as an inline skate or the head of dentist's drill, allowing the object to glide over them with incredible ease and speed. Unlike wheels, they don't turn on an axel; they turn on themselves.
You can see this in action with some great cut-away pictures of bearings.
The balls or rollers spin on themselves inside the bearing, reducing friction for the machine parts attached to them. It's much neater than using a bucket of oil, especially in dental equipment, and significantly more reliable than hamsters on a wheel.