The case, or enclosure, is the box that houses the PC's internal components. All of the hardware, except for the peripherals, is housed inside the case. There are two common styles of cases: "desktop" and "tower." The desktop case usually sits under the monitor and is roughly the size of two or three shoe boxes side by side. The tower case stands upright on one end and is usually placed on the floor. Tower cases can be the same size as a desktop case but can range up much larger. Another, less common, case style is the "rack mount" case which slides in and out of an equipment rack.
A case accommodates the motherboard which is fastened in with screws or snaps. Because there are different sizes, shapes and configurations of motherboards (referred to as "form factors"), you must make sure that the case you choose will accommodate the motherboard you intend to use. Refer to the section about motherboards for more information on form factors.
Cases are often sold with the power supply, so you should also consider the power requirements of your PC when choosing a case. More information about this can be found in the next section about power supplies.
Note that a PC should not be operated with it's case open. A good case is engineered to maximize cooling airflow while protecting the equipment from damage. If the case is not properly closed, the cooling operation of the case may be negated resulting in overheating. It will be exposed to more environmental pollutants such as dust and debris and the components are at greater risk of damage from accidental contact, bumps and spills.