A "driver" is a piece of software that controls or drives an item of hardware - Disk - Keyboard - Screen - Wireless card - Printer etc.
Without the appropriate driver the item will not work.
The driver sits logically between the operating system and the hardware it talks to.
Drivers may be had freely from the web site of the equipment's manufacturer.
If your laptop is missing the driver for the wireless device for instance - you may be able to connect to the internet with a wire - download and install the appropriate wireless device driver and then switch to wireless connectivity.
Some driver software comes built into the operating system - but not everything - the OS will not know which printer you might be using.
Rebuilding the OS will usually destroy any additional drivers that have been installed prior to the rebuild.
Peccavi, February 2011
