Bearing

a race of ball bearings.

Bushing, like these, reduce friction, at lower speeds, which do not require ball bearings

The term bearing is used in mechanical devices, to protect surfaces that move, relative to one another.

A common type of bearing is the ball bearing. Ball bearings are widely used in engines, where one rotating part is separated from the rest of the engine by a race of ball bearings. Rather than the two surfaces rubbing against one another they only make contact with the surface of the ball bearing. Rather than have its surface abraded the ball bearing rotates.

Simpler kinds of bearings, made of plates or tubes of soft metal, are called "bushings", which are useful when two surfaces do not move at a high enough speed to require ball bearings.