what does ATC mean, im assuming its some type of cardstock/paper card

ATC = artist trading card

ATC – 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 or through the mail, in person or online. The only official rule for ATCs is the size: 2.5″ x 3.5″. As a general rule, 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. You may find that individual swaps on Swap Bot have rules as to how the ATC should be created for that particular swap. Each one is different, so make sure you read the swap description carefully before signing up. There are some artists who choose to sell cards. Cards that are sold are called Art Card Editions and Originals (ACEO) and are often found directly on the artist’s site or on EBay.