ATC stands for Artist Trading Cards. Using what media you want to (collage, painting, clay, metal, drawing, scrapping) you create a card the size of a playing card (the dimensions are exact, I *think* 2.5 by 3.5) creating art and pictures on your card and you exchange them with other artists. No money is exchanged, they are not bought, they are traded. I think you can find other people to do it with over the internet. I am researching.
Then there is something called mail art. Mail as art, essentially. I guess decorating the mail you send out Is in fact considered art. How cool! I love to do that anyway, put rubber stamps, stickers and draw on envelopes. There's a whole world out there!:)