PS2 Type Keyboard Plug
The PS2 (also known as PS/2) is a round plug that is much smaller than the AT type keyboard plug. Most PC computers still support the PS2 type plug with a socket on the back of the computer box. This is probably the most popular plug used on PC keyboards today, but the trend is already starting to move to the USB type connector. Mac computers are not compatible with the PS2 type plug. Newer laptop computers are not compatible with the PS2 type plug.

USB Type Keyboard Plug
The Universal Serial Bus (USB) keyboard plug is one of the newest developments in keyboard design. This plug is a small flat device that fits in the USB port on the computer box or on a USB expansion hub. Many PC computers now feature multiple USB ports on the front as well as the back of the computer. The USB port system will support many different types of USB devices including keyboards, printers, external hard drives, etc. The USB port on a PC keyboard is compatible with a Mac computer. 

AT Type Keyboard Plug
The AT type keyboard plug is also known as the XT or LPX plug. It is not used on newer computers. It is a round plug approximately one half inch in diameter with 5 pins. Mac computers are not compatible with this type of plug.

PS2 to USB Keyboard and Mouse Adapter Plug. 
This plug adapter allows you to connect a PS2 keyboard (such as the keyboard above) and a PS2 mouse to a USB port on a computer. It is excellent for newer laptop computers since it allows you to connect both a PS2 keyboard and a PS2 mouse to one USB port. More Information

Price: $9.95

Mouse USB to PS2 Adapters
Many mice come with an adapter plug which allows you to plug a PS2 mouse into a USB port (or a USB mouse into a PS2 port). These adapters will not work with a keyboard. Keyboards use a much more complicated signal technique that requires an adapter that is specifically designed for keyboards.