Art in your Pocket: ATCs

Oh, Ok... So like a little picture?

Wikipedia has a pretty good general description and a list of links to other related sites.

Basically, ATC stands for artist trading cards. The format is 2.5x3.5 inches. You take that size format and create a piece of art using your preferred medium. Then you are able to trade them with other artists and form a collection. It was started as a way for artists to get their work out there to the general population a little easier and it's exploded since then.

That answers my question. :)