A bearing is a device to let fixed directional motion between two parts, typically rotation or linear movement. Bearings may be broadly grouped by the way they work. The most common of the many types are plain bearings and rolling-element bearings.
The two main types of rolling-element bearings are ball bearings and roller bearings. In ball bearings, the rolling elements are spherical balls. In roller bearings, the rolling elements are cylindrical, taper or spherical rollers. Ball bearings are shown in the pictures.
Bearings are used in machines that need to spin or move separately, like the rotor in a wind turbine. Egyptians used a form of roller bearing to build the Pyramids.
