ATC stands for artist trading cards. They’re basically mini works of art. The only rules are their size (they should measure 2.5 by 3.5 inches) and that they’re meant to be traded, not sold. Which just means that people who want to sell theirs call ‘em by a different name: ACEOs, or art card editions and originals. XD
If you have any interest in trading or buying any of my ATCs/ACEOs, feel free to drop me a comment!