Casters are a general term, including movable casters, fixed casters and movable casters with brakes. The movable caster is also called the universal wheel, its structure allows 360-degree rotation; the fixed caster is also called the directional caster, it has no rotating structure and cannot be rotated. Usually two kinds of casters are used in combination. For example, the structure of the trolley is two directional wheels in the front, and two universal wheels in the back near the push armrest. http://www.zinhardware.com/casters/