Shape - advantage is usually gained through the use of spheres or rollers
Material - the nature of the bearing material used is exploited. (An example would be the use of plastics which have low surface friction.)
Fluid - the low viscosity of a layer of fluid, such as a lubricant or as a pressurised medium, is exploited to keep the two solid parts from touching.
Fields - electromagnetic fields, such as magnetic fields, are exploited to keep solid parts from touching.
Combinations of these 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. Bearings vary greatly in terms of the forces and speeds that they can support. The maximum speeds that rotational bearings can support vary. Generally speaking, performance is defined in terms of the product "DN" where D is the diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute (rpm).
A bearing is a device to permit constrained relative motion between two parts, typically rotation or linear movement.
A linear bearing 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 centre, 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.
Essentially, a bearing can reduce friction via its shape, its material, or the introduction of a fluid between the two surfaces.