ATC's are Artist Trading Cards, miniature pieces of art. The only requirement is that they are always 2.5" x 3.5". They are most often traded amongst groups of artists. You can find the history of ATC's, and some gorgeous art here. At the PaperArtistsOnline forum, we often have themed trades for ATC's. As this was a 2 year anniversary for our forum, the theme was Celebration. 

These are the cards I received in exchange for six cards that I made. The last card in the photo is mine. If you click on the photo, it will enlarge, if you want a closer view. As you can see, there was a wide variety of views on the topic of Celebration, as well as many techniques.

To create my celebration party hat, I used a painted background that I already had on hand. The little flowers and tassel were made from crepe paper. I actually made these on January 1st, so a party hat seemed fitting.