A computer case, also known as a computer chassis, tower, system unit or a cabinet, is the enclosure housing most of the components of a personal computer. This typically doesn’t include the display, keyboard, and mouse. Cases are usually made from steel or aluminium. Other materials used include plastic, glass, wood and even Lego bricks in some home-built cases. The size and shape of a computer case depends on the form factor of the motherboard, as it is the largest component in most computers. Usually, computer cases include sheet metal enclosures for a power supply unit and drive bays and a rear panel that has room for peripheral connectors protruding from the motherboard and expansion slots. Most cases also have a power button or switch, a reset button, and LEDs to indicate power, hard drive activity, and network activity in some models. Some cases have built-in I/O ports (such as USB and headphone ports) on the front of the case. Such a case usually has the wires required to connect these ports, switches and indicators to the motherboard. 
It is important to note that the power supply unit (PSU) mounting point can vary from case to case. That said, the most common locations are the top of the case, which allows the PSU’s built-in fan to act as an auxiliary exhaust fan and the bottom of the case, with an often filtered vent that in turn allows the PSU to draw cool air from the outside.