The inner ring of a roller bearing is fastened to a rotating shaft. The outer ring is usually fixed to a stationary machine part (indicated with a transparent cylinder in this Demonstration). Alternatively, the inner ring may be stationary, and the outer ring rotating.
The rollers run on the outside surface of the inner ring and also on the inner surface of the outer ring. The axes of the rollers follow a circular path (called the pitch circle), while all other points of the rollers follow various hypocycloids.