Ball bearings is kind of rolling bearings. They allow very high rotating speed.
The following four parts usually constitute the ball bearing construction:
Outer ring: it in general does not move in case of rolling.
Inner ring: it rotates with the shaft.
Balls: they are mounted between the inner and outer rings, enable the movement between them.
Cages: they are holders for the balls to retain them in the right positions. Usually they are made of brass or steel.
Shields or seals: they are mainly functioned to prevent contaminant. Those bearings are well-lubricated with grease ex work.