I like the ATC basics provided by Joumana Medlej. She explains it best.
"As their name indicates, ATC are collectables, a brilliant idea born of the older sports-themed trading cards. The one rule that makes an ATC derives from their origins: the dimensions of the ATC must be 2.5"x3.5", or 64x89mm.
To this rule are appended a couple of conventions. First, an ATC mustn't be sold, only exchanged, as the whole essence of these tiny works of art is about artists meeting (by correspondence or online if need be) and exchanging their works, thus meeting many artists and getting exposed to many personal styles. Second, on the back of each ATC the artist writes part or all of the following information: name, contact information, title of the ATC and number (1/8, 2/8...) if it's part of an edition. By definition ATCs are made in limited numbers, often no more than one of a kind. Unique ATCs are called originals; sets of identical ATCs are called editionsseries. Don't be intimidated by the concept of small editions or originals: very few people are anal about this. What most collectors really want are cards that were made with care. Based on that, numbers are meaningless."
Her site was the first place I found when I was looking for information about ATCs and I return there to admire her work. Her site is extensive, covering her travel journals, her publications, her art, photography and more.Here are a couple ATCs next to a couple subscription card journal pages for size comparison.
I've not made many ATCs but I will. There are just so many ways to get my art journal grove on. Can't keep up with them. Just fall into one grove or another when I get down into my office. Lately, I've been carving my own rubber stamps and can't get enough. I might move on to carving wood soon! I never thought I'd enjoy carving but I really dig it.