PS/2

From Grokdoc

PS/2 ports are one way of connecting a mouse and keyboard to your computer. Most new computers use USB instead -- you can tell if you have PS/2 or USB by looking at the plug at the end of your mouse or keyboard cable. If it is round, you have PS/2. If it is a small, flat rectangle you have USB. Some very old computers may have yet another connector: the D-Sub (or serial) style plug. If your mouse cable connector has two straight rows of tiny pin holes (five holes in one row and four in the other) you have one of these.

The tricky thing about using PS/2 is that you have to plug the mouse and keyboard into the right ports on your computer: each port is exactly the same size and shape, but they are usually different colors. The mouse port is usually green, and the keyboard is usually purple. If you start up your computer and it tells you that it can't find the keyboard, try switching these two around and restarting.