I/O Ports

The average desktop computer has anywhere between 10-20 I/O ports, of varying sizes, shapes, and colors. It can be quite challenging to hookup a computer for the first time.

Below are two pictures with the various I/O ports labeled for you.

Fig 1: Dell Precision 340 I/O ports.

Fig 2: Apple PowerMac G5 I/O ports.

Power

The primary power interface for PC's has not changed much in the last 20 years. Computers require a grounded (three prong) power outlet in order for proper operation.

Parallel (DB-25)

The parallel port, also known as a DB-25 port, is primarily used for printers (occasionally scanners), and is often referred to as the "printer port". In the 80's and 90's it was also used for networking or even for external storage drives. The parallel (or printer) port is quickly on its way to being depricated, as most printers nowadays are using USB as their interface. Newer computers are starting to be produced without them, and likely in the next 2-4 years the parallel port will be no longer be used.

Serial (DB-9)

The serial port, also known as a DB-9 port, is not really used on modern computers. The main reason for its continues existence is to support really old hardware. In the 80's it was used for all kinds of peripherals but by the 90's it was used primarily for mice, joysticks, and modems. The serial port is pretty much already depricated, but since it shares motherboard components in common with the parallel port, it is still hanging around on most desktop machines. Laptops rarely are produced with them, and likely in the next 2-4 years the serial port will be no longer be used at all.

PS/2

The PS/2 port was developed by IBM to connect keyboards and mice. Computers will usually have 2 of these ports, one purple (for the keyboard) and one green (for the mouse). PS/2 was the standard for close to 15 years, but is now being replaced by the more versitile USB port. Many newer computers no longer come with PS/2 ports.

USB

The USB (Universal Serial Bus) port is the most versitile interface used on modern computers. Via USB, you can attach mice, keyboards, joysticks, printers, scanners, cameras, external hard drives, external cd and floppy drives, and even hook up to a digital video recorder or a even network switch. USB is intended to be the jack-of-all-trades port for a computer. There are two speed specifications: USB 1.0 and 2.0. You can plug USB 1.0 and 2.0 devices in either speed port, but if you plug a 2.0 device in a 2.0 port, it will operate at a much higher speed. There are also 4 form factors for the USB ports: USB A, USB B, Mini-A, and Mini-B. Both of the computers above have USB A ports.

More information than you could ever need:
http://en.wikipedia.org/wiki/Universal_Serial_Bus

Firewire (IEEE 1394)

Firewire (aka IEEE 1394) was developed by Apple Computers (with help from other vendors) in 1995. The term "Firewire" is trademarked, so other companies have devices that utilize the same technology, but under different names. Firewire is used for lots of devices, similar to USB. Firewire was much faster than its contemporary, USB 1.0, but has similar speeds to USB 2.0. Firewire never reached the popularity level of USB, and has since been dropped by Apple on iPods (but not on their computers yet).

Network/Ethernet (RJ-45)

The port used for networking on most modern computers has many different names: RJ-45 Jack, Network Port, Ethernet Port, Cat5, or Twisted-Pair. Most modern computers are hooked up to ethernet networks, using category 5 twisted pair network cables, that plug into RJ-45 jacks. Hence the confusion over the name of the port.

Modem (RJ-11)

While the modem was the primary way to access networks in the 80's and 90's, dialup (and therefor the modem) is in the process of being replaced by broadband (cable and DSL). Many modern computers do not come with modems by default.

Sound

Most computers manufactured over the past 15 years come with the capability for sound. Often there are 3-5 sound ports (mini-din) that are used for various things like: headphones, speakers, or microphone. Often the ports will be color-coded and have little icons that represent their usage. Some newer machines come with the capability to use an optical connector for sound. The Dell Precision 340 pictured above has 3 mini-din sound ports that are color coded, while the Apple Macintosh G5 has 2 mini-din connectors and 2 optical connectors.

VGA (HD-15)

The VGA (Video Graphics Array) port is used exclusively for video, and has been the main PC video interface for 15+ years. The VGA port is quickly being replaced by DVI, though you can get DVI to VGA adapters.

DVI

The DVI (Digital Video Interface) port is used exclusively for video with newer monitors. There are multiple proprietary versions of DVI, that have a slightly different form factor or different pin layout. Make sure to compare the end of the DVI cable to the port to make sure they are compatible. In the Picture of the Apple Macintosh G5, there are 2 DVI ports: one rounded (proprietary) and the other squared off (standard).