I've been making ATCs for a few years and although the idea is to swap them, I do tend to keep mine. They are generally made from a base of card (well mine are anyway), but can be made from anything from fabric to clay to glass. There are two main rules with ATCs; the size should be 3.5" x 2.5" and they should never be sold, only swapped. When entering into a swap whether at a networking event, through a magazine or swap website, artists will sign, date name and number each card and provide an email address.
One thing I like about ATCs is that I get to use stamps and images I wouldn't normally use for making cards. A mini canvas, I tend to start with a background, have a central image or a collection of images and some embellishments. These two are my most recent ATCs and are a very simply made using a collection stamped images (in a single colour), glossy vintage images, metal charms and ribbon.