Bearing is a device to permit constrained relative motion between two parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.

There are at least six common principles of operation:

SLIDING BEARINGS , usually called  BUSHES ,  BUSHING , JOURNAL BEARINGS , SLEEVE BEARINGS, RIFLE BEARINGS, or  PLAIN BEARINGS .

ROLLING-ELEMENT BEARINGS such as ball bearings and roller bearings

JEWEL BEARINGS in which the load is carried by rolling the axle slightly off-center

FLUID BEARINGS  in which the load is carried by a gas or liquid

MAGNETIC BEARINGS , in which the load is carried by a MAGNETIC FIELD  

FLEXURE BEARINGS in which the motion is supported by a load element which bends.