What are A.T.C.s?
The letters stand for Artist's Trading Cards. The idea is that you make little pieces of art that are the size of a standard trading card, like a baseball card, 2.5"x3.5". These cards are only traded with other artists, never sold. Its a trend that started in the 90's with live trading card shows for artists, just like any other card collectors. Now they are traded avidly over the net as well. You can see many examples on photo sites like Flikr and endless examples will pop up if you enter "ATC" into google's image search engine.