ATC stands for "Artist(ic) Trading Card". It's a 2.5"x3.5" handmade trading card that can be created using any artistic format you like.
I do mine in Scrapbooking style, since I cannot paint or draw.
The theme for this particular swap is "Sepia", and I had to make 4 cards. I then send those cards to the swap host, who distributes 3 cards each to everyone that has participated, and keeps 1 card from each person for herself.
On the cards I used print outs of old sepia photographs (one of my great-grandparents on their wedding day, and the other a Victorian photo I found online), a sepia coloured picture of the Houses of Parliament in London, and the last one (which kind of came out more orange than sepia) was a forest photo I also found online.