"A bearing is a machine element that constrains relative motion between moving elements to solely offer the desired motion". Bearings are designed to provide free linear movement of the moving elements or free rotation around a fixed axis or it may prevent motion by controlling the vectors of normal forces that acting on the moving element. They are mainly used as friction-reducing devices between moving parts.
When a metal contacts with another metal produces a large amount of friction. Due to this friction the metals wear and tear, produce grinding that slowly degrades the metals that are in contact. Bearing reduces friction by allowing one moving part to glide past another moving part. Bearings consists of a smooth metal ball or roller that rolls against a smooth inner or outer metal surface (race). The balls or rollers take up the load, allowing the device to spin.