The Motherboard and System Unit

The system unit is the box in which the main components of a computer are stored. For a desktop computer, this is typically the box that contains everything except the keyboard, mouse, screen, printer and other peripherals. Some desktop computers, such as the iMac, do not have a system unit but instead house all the system unit components in the same case as the screen. The system unit for a notebook or tablet computer is the part beneath the keyboard. The typical system unit is the place where the following items are housed:

Motherboard

A PC motherboard is a printed circuit board used in notebook and desktop computers. It is also known as the mainboard or logic board. It contains places to attach the various components: CPU, memory, hard drive controller, graphics card, sound card, keyboard, and mouse. It probably also contains USB, firewire, serial, parallel and possibly other ports.

Power Supply

The power supply supplies power to the components of your computer. (Isn't technology easy to understand -:)?) It converts AC power to low voltage DC power and supplies the various voltages needed by the various components of the computer. In desktop computers the power supply is contained in a box approximately 6"x6"x3", which also contains a fan to keep the power supply cool. In notebook computers the power is supplied by the battery. There is a converter inside the notebook to provide the correct voltages for each of the components. The battery is charged by an external power supply sometimes referred to as a brick, because it contains a brick shaped box with one wire going to the outlet and the other plugging into the computer.

There is also an external device called an uninterruptable power supply, or UPS, that your computer plugs into. It contains a battery that is charged from the power cord into the main outlet. It has several outlets on it that supply power to various components. It is similar to a power strip/surge protector except that the battery will allow your computer to continue running for a brief time when the household power goes off. This will allow you to shut down your computer in a timely manner if the main power is lost. A home UPS will typically have enough power to run a desktop computer and monitor for at least 10 minutes after power is lost.

Expansion Cards

Expansion cards can be plugged into the motherboard to provide extra features that your system does not have. Desktop computers normally have from two to six expansion slots depending on the size of the system unit. Most modern motherboards have many capabilities built into them so the need for expansion cards is not as great as it used to be. Probably the most common use of expansion cards is for high-end graphics. The typical graphics capability that is included on modern motherboards is quite adequate for most applications but is perhaps not good enough for three-dimensional animation used in advanced computer games. Most motherboards also do not support RAID or external SATA without the use of expansion cards. TV tuners are also typically not included in most computers and may need expansion cards.

Ports

In computer hardware a port refers to an interface between the computer and other computers or peripheral devices. Common ports include:

Further Info

howSTUFFworks How PC Power Supplies Work
Wikipedia Computer Power Supply
Wikipedia Expansion Cards
Wikipedia System Unit
Wikipedia Port (hardware)
Wikipedia Universal Serial Bus
Wikipedia IEEE 1394 Interface (firewire)
Wikipedia PC Motherboard