PS/2 is a type of port used by older computers for connecting input devices such as keyboards and mice. The port was introduced with IBM's Personal System/2 computer in 1987 (which was abbreviated "PS/2"). In the following years, the PS/2 port became the standard connection for keyboards and mice in all IBM compatible computers.
The PS/2 port has six pins and is roughly circular in shape. Since each PS/2 port is designed to accept a specific input, the keyboard and mouse connections are typically color-coded. For example, the keyboard port on the back of the computer is often purple, while the mouse port is usually green. Similarly, the connector on the end of the keyboard cord is purple and the mouse cord connector is green. This makes it easy for all users to know where to plug the cables into the computer. The concept is similar to the color-coded composite audio/video connections on the back of a TV, which use red, white, and yellow connectors.
While the PS/2 port enjoyed a good run for almost two decades, now most keyboards and mice use USB connectors. Unlike PS/2 ports, USB devices can be plugged into any USB port or even a USB hub and the computer will automatically determine what the device is. USB is also "hot swappable," meaning the connections can be removed while the computer is running. If you remove a PS/2 device while the computer is on, it may potentially cause damage to the hardware. Therefore, if you are using a PS/2 device, it is best to turn off the computer before connecting or unplugging a keyboard or mouse.
NOTE: The term "PS2" is also a common abbreviation for Sony's PlayStation 2 game console.