the shape of the neck of the guitar on the backside where your fretboard isnt itself
C-shaped are pretty much a semi-circle. D-shaped necks have a flat spot on the back, and arent too common.
I've got an old semi-hollow from the '60s that has a D-shaped neck (not original, but same era). I love D necks now. I've also got a couple '60s guitars with really fat C necks...don't like those necks much...