(1) A loudspeaker. The terms "driver" and "speaker" are used synonymously; however, "the speaker has four drivers" means that the speaker enclosure contains four speakers.
(2) A hardware device (typically a transistor) that provides a signal or current to trigger some function. See line driver.
(3) Also called a "device driver," it is a program routine that links the operating system to a peripheral device. Written by programmers who understand the peripheral hardware's command language and characteristics, the driver contains the precise machine language necessary to perform the functions requested by the application.
The Driver Is the Interface
When a new peripheral device is added to the computer, such as a graphics card, its driver must be installed in order to use it. The application calls the operating system, the operating system calls the driver, and the driver makes the device work (it "drives" the device). Routines that perform internal functions, such as memory managers and disk caches, are also drivers.
Drivers Can Drive You Crazy!
Sometimes, the solution to a problem is to update a driver, because the current version has bugs. Hardware vendors are always releasing newer versions. For example, everything you see on screen is the result of the display driver (video driver) drawing the screen according to the commands that the operating system issues to it. Such drivers are extremely complex and can be error prone.
There Used to Be More Drivers
In the "old" days of DOS PCs, in order to provide complete control over the printing of a document, software applications included a driver for each of the popular printers. With Windows and Mac, the printer driver is installed into the operating system, not into each application. All applications print through the OS, which commands the printer via the one driver for that printer. See filter driver and driver signing.
Windows Display Driver
To display something on screen, a Windows application makes calls to the Graphics Device Interface (GDI). GDI converts the calls to driver functions, and the driver commands the display adapter (graphics card).
Every peripheral device requires a driver. The drivers provide the detailed instructions necessary to activate and control the device.