Dictionary.com states that stacks are "a more or less orderly pile or heap." Below are some example of stacks.
(Pictures taken w/ my G1.)
Coupons - The red ones are a little low. I usually give those out once I get them.
Money - This is just a baby example of stacks ($600). These are stacks of 1's and 5's.
I tossed in a bill just for kicks. (Most of you probably have 20's and higher.)
*This post was not in any way intended to showoff or brag about what I have. Honestly, all that money isn't even mine (except for the $100).