Castors are a general term that includes movable casters, fixed casters, and movable brake casters. The movable casters, also known as universal wheels, have a structure that allows for 360 degree rotation; Fixed casters, also known as directional casters, have no rotating structure and cannot rotate. Usually, the two types of casters are used in combination, such as the structure of a handcart with two directional wheels at the front and two universal wheels near the push armrest at the rear. Castors come in various materials, such as PP casters, PVC casters, PU casters, cast iron casters, nylon casters, TPR casters, iron core nylon casters, and iron core PU casters.