The basic function of any ball bearing is to reduce friction between the two elements because rolling things have less friction than the sliding things and the reason is not hard to fathom. If things were to slide instead of roll the entire surface of the two things sliding against each other would be in touch which means more friction and more resistance.

When things are rolling they have less friction because of the fact that they have less amount of surface in contact and that means that less friction and resistances.

The bearings then actually have an inner surface to roll over and outer surface to roll over. These balls then get most of the load and but of course then there are various kind of ball bearings for different applications.

Roller ball bearings have three components namely the outer race and the inner race. Balls get the load from the outer race and transfer the load to the inner race. But roller ball bearings can only handle small amounts of load since the balls have less area of contact to the races.

Roller bearings are a variety of ball bearings in the sense they do not have the balls but a cylinder where the bearings come in contact over a large area and that means that they can in fact handle larger loads. The variety used in cars is the tapered roller bearings and that means that they can in fact handle larger loads and also they are used in pairs so that the load can be handled in both the directions.

Nowadays a lot of ceramic ball bearings are being used since they are lighter than steel and generate less heat and hence can go faster than steel. The ceramic material can be silicon nitride or silicon carbide or zirconium.

Again you can have entire ball bearing built out of ceramic material or you can have only the balls of the bearings built of ceramic.