The term "bearing" is derived from the verb "to bear"; a bearing being a machine element that allows one part to bear (i.e., to support) another. The simplest bearings are bearing surfaces, cut or formed into a part, with varying degrees of control over the form, size, roughness and location of the surface. Other bearings are separate devices installed into a machine or machine part. The most sophisticated bearings for the most demanding applications are very precise devices; their manufacture requires some of the highest standards of current technology.

Bearings reduce friction between two objects, which allows moving parts to run smoother. Bearings are divided into two main categories: thrust bearings and radial bearings. Thrust bearings support axial loads, which are parallel to the axis of rotation, while radial bearings support turning shafts perpendicular to the axis of rotation. Depending on the application, there is a wide range of bearings to choose from. Common types of bearings include ball bearings, split bearings, and roller bearings. Bearings are also available in mounted, unmounted, and flange styles. Ball bearings are the most common type of bearing and are used in applications requiring support of smaller loads. Split bearings are ideal for hard-to-reach places and quick installation while roller bearings are suitable for heavier loads. Roller bearings are generally available in three styles: tapered roller bearings, cylindrical roller bearings, and spherical roller bearings. Key elements to consider when selecting bearings are outer diameter and the shaft diameter the bearing will fit.