Drivers:

Drivers are similar to system extensions, but they load during a different part of the startup process. Drivers are the small programs that act as an interface between a piece of hardware and the rest of the computer. Disk drives have drivers, and keyboards and mice have drivers, and CD ROM drives have drivers. Just about any piece of hardware you can add to a Macintosh has a driver associated with it. Some of these drivers are brought into the machine from the piece of hardware during startup, and others are in the System Folder on the startup disk.