Simply put, an Artist's Trading Card, or ATC, is a 2.5" x 3.5" card that's been altered, collaged, painted, drawn, or otherwise "arted up". The artist's name and contact info, as well as the title of the piece is on the back. These are then traded, much as people would trade sports trading cards.
Here's the rule: anything goes. Most prefer to keep things relatively flat so that they'll fit in a regular sports card binder or sheet protector sleeve, but the medium, content, and creativity is all yours. Just make something and get ready to trade.
Follow this link to get to some of my own ATCs.
Wanna trade? Here's my basic ideas about what I'm looking for in a trade, and my address.