A driver (also known as a 'device driver' or 'software driver') is a piece of software that lets your operating system communicate with your hardware. Devices like keyboards, mice, speakers, and printers all use a software driver to interact with the operating system and other software applications.
When you purchase an operating system such Microsoft Windows XP, many device drivers are built into the operating system. However, if you buy a new type of hardware device that the operating system did not anticipate, you'll have to install a new device driver. A device driver essentially converts the more general input/output instructions of the operating system to messages that the device can understand.