GPU stands for graphics processor... in the olden days the GPU only put the 2D image the OS gave it on the screen, and when games were played it took intructions from the game,and then "drew" a set of shapes on the screen, these shapes formed the graphics of your game, the reason the CPU doesnt do this is becuase its not designed to do it, and a dedicated processor can do it much much better and much much faster. The GPU memory is used to store the instructions for the GPU and the results of the processed intructions, and sometimes bitmap images as well.
With later versions of the Mac OS (I think 10.2 maybe 10.3) the GPU no longer does 2D graphics anymore, the OS sends 3D instructions to the GPU to draw the GUI, this allowed for a overall increase in speed as the CPU was now left with less to do, with Tiger (10.4) Apple created a freamwork for developers called Core image that allowed them to write programs that could use the more graphics optimized features of the GPU, in effect have faster graphics effects, a well as have lower CPU usage. I think Aperature uses Core Image to do most if not all of its graphic/image manipulations, so the manipulations run very fast because the GPU does some work and the CPU as well, the GPU memory is used to store your manipulated images...
Very long worded but I couldnt think of a better way to describe what a GPU does in relation to Aperature... hope it explains it!