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A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. 

The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. 

Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.

Common motions permitted by bearings are:

• Radial rotation, e.g. shaft rotation;

• Linear motion, e.g. drawer;

• Spherical rotation, e.g. ball and socket joint;

• Hinge motion, e.g. door, elbow, knee.

Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.

• Shape: gains advantage usually by using spheres or rollers, or by forming flexure bearings.

• Material: exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)

• Fluid: exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.

• Fields: exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.

• Air pressure: exploits air pressure to keep solid parts from touching.

Combinations of these can even be employed within the same bearing. An example is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.