The Gertboard is probably best described as an IO development board.
It contains a number of useful components such as buffers, motor controller, darlington drivers and A/D, D/A convertors. These components can be connected to the Raspberry Pi GPIO connectors using jumpers. You can then connect the board to things like relays and motors so that they can be controlled using the raspberry Pi.
See the video http://www.raspberrypi.org/archives/868
I would suggest that it is not really aimed at the complete beginner but I'm sure you will find help in the forums here. For the beginner I think the PiFace is probably more suitable, it is more limited in the options available but is basically plug-in and go.