It is a new grown-up card collecting craze. A global art happening and it is call ATC exchange. We have stampers, scrapbookers and other artist that do this. There are classes for beginners. There are group swaps.
B. no thicker then 1/2 cm with embellishments.
C. open theme ( some times there is a one theme like on Thanksgiving)
D. put contact information on back of the ATC (sometime that is optional.)
For every card you send you get one back.
As you can see I keep mine in a large note book. Fun Fun. Click on picture to enlarge and see details of the cards.