Taken from Wikipedia and StrathmoreArtists.com:

What are artist trading cards?
Artist trading cards (ATCs) are miniature pieces of art that are traded around the world. Artists create, trade and collect art at organized “swap” events, either in person or online. The only official rule for ATCs is the size: 2.5" x 3.5".

How do I create ATCs?
Most swaps are open to any media, materials or techniques as long as the card fits into a standard trading card sleeve. ATCs are traded, not sold. However, there are some artists who choose to sell cards. Cards that are sold are called Art Card Editions and Originals (ACEO).