Fans of the Gertboard claim that the device is the ultimate expansion board for the Raspberry Pi. Having spent quite a bit of time with the Gertboard, I can tell you that there is indeed substance to that claim.
Gert van Loo (pronounced van LOW) is a Broadcom engineer who also happens to be one of the principal designers of the Raspberry Pi. As I’m sure you can correctly guess, Gert also invented the Gertboard.
Formally defined, the Gertboard is an expansion board, also called a daughterboard, that connects to the Raspberry Pi GPIO headers and gives you instant access to a tremendous variety of input/output options.
You can look at the Gertboard as an activity center or toybox with which you can experiment ...