An artist trading card (or ATC for short) is a tiny, original piece of art created with the intention of swapping or trading it with another artist, not selling it. The one rule an artist trading card must adhere to is the size. An ATC must be 2.5x3.5 inches or 64x89 mm. (Why? That's the size of the original collectible sports trading cards.)
On the front of an ATC an artist creates an original work to showcase their art. It can be a one-off, part of a series, or a limited edition. On the back the artist puts their name, contact details, title of the ATC, number if it's a limited edition, and sometimes the date it was created.
Who you trade with and whether you trade one of your card for one of someone else's, or think it's more valuable and want multiple cards, is up to you. You can also trade cards you've received if you don't want to keep them. The whole aim is to be creative and communicate with other people being creative.
Regular trading sessions are organised in larger cities and although face-to-face trading fits the original intention of artist trading cards better because you're meeting new people, trading by post also happens. You can find other ATC creators through groups such as the Flick Artist Trading Cards. An international postal trade is organized by Copy Left in which you submit 20 cards and get a mixed set back.
An ATC created with the intention of selling it is known as an ACEO (short for Art Cards, Editions and Originals). ACEOs are often sold on EBay. Why the two names when the only difference is that the one is sold and the other not? Well, neglect that difference and you could find yourself in the middle of an argument between someone who believes all art should be for sale and that ATCs exclude non-artists, and someone who believes art cards is a distinctive form of art and shouldn't ever be sold.
Buy Direct: Ready-cut Card for Artist's Trading Cards
See Also: Making Artist Trading Cards Digitally (From About.com's Desktop Publishing Guide)