Drivers are the programs that allow your OS to communicate with the given hardware in your computer.
Depending on your OS, it will either need "Real-Mode" or "Protected-Mode" drivers to allow communication between it and the piece of hardware.
Real-Mode is the term that is most often given to MS-DOS nowadays. Not all MS-DOS sessions are really considered "real-mode."
Protected-Mode is a relatively new term that has been coined for Windows95/NT OS's.