The system unit, also known as the computer chassis or tower, is the main component of a desktop computer. It is the rectangular-shaped case that contains the majority of the computer's internal components, such as the central processing unit (CPU), memory, and storage devices. It also houses the motherboard, which is the main circuit board that connects all of the internal components together.
The system unit typically has several external ports and connectors, such as USB ports, audio jacks, and network connectors, which allow the user to connect external devices to the computer. It also has a power supply unit (PSU) that converts the alternating current (AC) from the wall outlet into the direct current (DC) that the computer's internal components require.
The system unit also has several internal bays for storage devices such as hard disk drives (HDD) and solid-state drives (SSD), expansion slots for additional components such as graphics cards and sound cards, and cooling systems such as fans to keep the internal components from overheating.
In summary, the system unit is the main physical component of a desktop computer that houses the majority of the internal components and connects them together. It also has external ports and connectors to allow the user to connect external devices, as well as a power supply unit and cooling systems to keep the internal components running smoothly.