from my buddies at GunPros.com:
“Single stack” and “double stack” are terms used to describe the way in which ammunition is fit into a magazine. Single stack magazines layer rounds in a single vertical row, while double stack magazines hold two vertically staggered rows packed tightly next to each other. Double stack guns, therefore, have the advantage of holding more ammunition. Single stacks carry less ammunition, but are obviously less wide, which makes them generally lighter, thinner, and more easily concealed.