The term "input device" merely refers to the instrument or hardware used to communicate with the computer. In effect, it refers to whatever device is used to put information into the computer's hard drive, and includes the keyboard, mouse, trackball, scanner, microphone, and any other means of transmitting information or data to one's computer.
In contrast to the "input device," an "output device" is that part of a computer or computer system that is used to access the information or data. Types of "output devices" include the monitor on which information is displayed, the printer, which is, obviously, used to produce "hard copies" of the desired information, and the speakers that are integral to the computer or that are connected to a computer by cables and through which information is audibly communicated.
In short, the "input device" is that part of the computer hardware used to transmit data to the hard drive; the "output device" is that part of the hardware used to view or access the information stored on or streamed through the computer.
The function of an input device is to communicate information to a computer or other sort of information processing equipment. Input devices are types of peripheral devices that communicate with processing units.
The most common forms of input device for personal computers are keyboards and mice. A keyboard allows a user to input alphanumeric information to a computer, and is considered a discrete input device, as it only can input fixed or discrete types of information.
Another type of input device is a tracking device that communicates spatial information. Types of tracking devices include mice, trackballs, touch pads, and joysticks. Mice with buttons actually are hybrid devices, able to input discrete information in the forms of clicks or tracking information.
Other processing systems can be activated by sound. These might include sound activated devices or more complex forms of voice recognition technology.