Artist Trading Cards are miniature art works that are made for exchange between artists. They may be in any medium or technique, but the rules are that they measure 2.5 x 3.5 inches, that as art works they are individual and unique, or at the very least made in limited editions like prints rather than mass produced, and that they be traded, not sold. Additional conventions are that the artist will have signed, dated, titled and put contact information on the back, and while cards may be displayed like baseball cards in plastic sleeves, a custom-designed envelope may also be made.
Their origin has been credited to Swiss artist M. Vänçi Stirnemann, who organized trading sessions in Zurich in 1997(http://www.artist-trading-cards.ch). Trading is of fundamental importance to ATCs, and so the card itself is both art object in its own right and the artifact of performance art.
Needless to say, it's an art form that is readily adaptable to haiga, because the small size of the card can easily accommodate haiku, and because ATC trading, like haiku, is a social activity. This past Spring, seven of us from WHChaikumultimedia decided to do an exercise with haiga ATCs. We hope you'll enjoy our little collection and encourage you to try them for yourself. In addition to the above website, you'll find a lot more information at http://www.cedarseed.com/air/atc.html