What is Printer Driver?

Legal Definition
In computers, a printer driver or a print processor is a piece of software that converts the data to be printed to the form specific to a printer. The purpose of printer drivers is to allow applications to do printing without being aware of the technical details of each printer model.

Printer drivers should not be confused with print spoolers, that queue print jobs and send them successively to a printer.
-- Wikipedia
Legal Definition
A program on the computer that allows softwares to send commands to the printer. The driver software is unique for every model and for every operating system and as such the correct version must be installed before the print engine can be used.