Plug and Play describes a special technology that gives users the ability to plug a device into a computer and have the computer recognize that the device is there. The user doesn't have to tell the computer. In many earlier computer systems, the user was required to explicitly tell the operating system when a new device had been added. Hard disks, sound and graphics cards, modems and other peripheral devices can easily be connected (plug) and the computer automatically knows that they can be used immediately (play).