Rolling Element Bearing / Sliding Bearing
The two most common principles of operation for bearings are rolling element bearings and sliding bearings. The main purpose in both cases is to permit relative motion between two members with low frictional resistance.
Rolling element bearings carry a load by placing rolling elements between an inner and outer member.
Sliding bearings carry a load by placing a thin film of lubricant between an inner and outer member. Alternatively no lubricant is required if one or both members are made of a low friction material.