A well has several strings: a conductor, surface casing, one or more intermediate strings, and a production casing or liner. Each setting depth is chosen so the next open-hole section has a mud-weight window, a pore pressure the mud can hold without exceeding the fracture pressure at the previous shoe.
Casing is not tubing. Tubing is the removable pipe inside the production casing that carries the flow. Casing, once cemented, is part of the well's structure. Its size is an outside diameter in inches, and its weight is quoted in pounds per foot.