UPDATE - Added a link to a new article from innowen at DIY Planner.
What are they?
Artist Trading Cards, or ATCs as they’re more commonly known, are a new way for artists to get to know each other and share their creativity. The only rules are that the cards must be 2.5″ x 3.5″ (6.35cm x 8.89cm), and the cards should be traded with other artists rather than sold - however, some cards are sold, and these are known as Art Card, Editions and Originals or ACEOs for short.