A 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.

An example of a ball bearing
Major types
Common motions include linear/axial and rotary/radial. A linear allows motion along a straight line, for example a drawer being pulled out and pushed in. A rotary bearing or thrust bearing allows motion about a center, such as a wheel on a shaft or a shaft through a housing. Common kinds of rotary motion include both one-direction rotation and oscillation where the motion only goes through part of a revolution. Other kinds of bearings include spherical bearings such as ball joints which are used in car suspensions and some computer mice.

Bearing friction
Low friction bearings are often quite important for efficiency and wear and to permit high speeds. Essentially, bearings can reduce friction by shape, or by its material, or by introducing a fluid between surfaces.
By shape, finds an advantage by reducing contact surface, such as using a sphere to roll anything on.
By material exploits the nature of the bearing material used. An example would typically be the various plastics that have self-lubricating properties.
By fluid exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurised medium to keep the two solid parts from touching.
Combinations of shape and properties can even be employed with the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.
Principles of operation
There are at least six common principles of operation:
• sliding bearings, usually called "bushings", "journal bearings", "sleeve bearings", or "plain bearings"
• rolling-element bearings such as ball bearings and roller bearings
• jewel bearings, in which the load...