Artist Trading Cards (or ATCs) are miniature works of art about the same size as modern baseball cards, or 2 ½ X 3 ½ inches (63 mm X 89 mm), small enough to fit inside standard card-collector pockets, sleeves or sheets. The ATC movement developed out of the mail art movement and has its origins in Switzerland. Cards are produced in various media, including dry media (pencils, pens, markers, etc.), wet media (watercolor, acrylic paints, etc.), paper media (in the form of collage, papercuts, found objects, etc.) or even metals or cloth. The cards are usually traded or exchanged rather than sold.
I received an invitation for the Oklahoma Visual Arts Coalition Annual Members' Meeting. The theme is 'game night' and attendees are invited to bring small works of art to be traded with other artists. There was an artist trading card template attached to the invite, which encouraged to make as many as you like... the more you bring, the more you can trade.
Needless to say, I've spent the entire day making cards :)