A device driver is a piece of software designed for a particular device (printer, mouse, monitor, or what have you) and the particular application program or environment you’re working with. The driver serves as a go between for the program (or environment) and the device, translating the software’s desires into commands the device understands.
For example, if you have a Laser-jet printer, you’ll need to have the Laser-jet driver to print. If you buy a new or different type of printer, you will then have to change to the driver that matches the new model of printer.