Melanie,
As WRAEZ said, there are several types of design walls and her explanation was right on target.
I have used a flannel sheet as one, as it held my blocks firmly enough for me to get a good idea of the placement that I wanted. They don't have to be fancy at all, just something to help you look at your blocks from a distant angle to get a better idea of how they'll look in your finished quilt.
Hope this helps.
Ty