We use input devices every time we use a computer. Simply speaking, it is these devices which allow us to enter information. Without them, the computer would not know what we want it to do.
Input devices have a history as long as computers themselves. Perhaps the first input device was the simple electronic switch ( similar to a light switch ) which turned bits on or off. There were hundreds or even thousands of these switches on larger computers. It used to take a team of programmers hours or even days to set up a computer to perform a single calculation.
Switches and jumpers are still used today on computers. For instance the power button on the computer is a switch which is also an input device telling the computer to power on or power off. Tiny switches called jumpers are also widely used on motherboards to change important settings such as processor clock speed or memory speed.
Most likely in front of you right now are two of the most popular input devices: the keyboard and the mouse. And instead of a mouse on a laptop computer you normally have a touchpad.
As computers evolved throughout the late 20th century, computers became more and more interactive. Input devices came and went. Some lasted and some did not. The light pen and the joystick are almost unknown today, although they were popular before the mouse and the gamepad became well-known. Touch screens are already replacing keypads on mobile phones and may come to replace or augment keyboards and mice on PCs and laptops in the near future.
Different people prefer different input devices for doing same task. For instance, many graphic artists prefer to use a stylus and graphics tablet rather than a mouse. It might offer them a greater deal of artistic freedom, or precision while performing their work.
Sufferers of carpal tunnel syndrome often prefer a trackball or stylus to a mouse. Handicapped computer users have invented a wide array of input devices designed to replace the mouse including devices controlled by foot or even eye movement.
Not only PCs and mainframes use input devices. Almost all computers feature some kind of input device. Special scanners are used in many stores and warehouses called barcode readers to enter stock and sell items at the cashier. These are input devices as well. Even microphones can technically be called input devices as a computer can repond to them and interpret them as incoming data.
Corporations and especially government institutions are already implementing the second generation of input devices to improve security. These include retina scanners and/or fingerprint readers to replace or improve accuracy of username and password authentication. You will be seeing more of this kind of biometric authentication in the coming years as a general remedy for weak passwords or leaked passwords.
In summary, input devices are how you interact with a computer. The computer responds to your input and hopefully does what you need it to do. It seems really simple, and that's the way it was meant to be!