Drivers in CC are pieces of software that read and/or write external redstone signals in parallel or serial transmission that go to another mechanism, other than the computer
Drivers are fairly hard to write, and involve use of pre-created APIs. Drivers can allow you to use/run external redstone devices from your computer, therefore allowing for many more possibilities thrown into the coder's arsenal.
Drivers may be used for serial transmission between computers too, but that leads us into networking. Drivers have one more large, universal capability, which is that they can be used on computers, on computers, creating a loop of dependancy, making 1 computer that is twice as strong.