Artist Trading Cards and Art Card Editions and Originals can be a quick and easy project. You can use them to try out a technique before doing a larger work, as something small and compact you can share with friends, etc. There are very few restrictions to keep in mind.
Artist Trading Cards (ATCs) should be given or traded, not sold. ACEOs can be sold.
Both should be reasonably flat, and are always 2.5 inches wide and 3.5 inches tall, so that they fit into standard trading card sleeves. They are often traded by mail, and the flatness also helps keep this easy.
They can be made in any media that fit these rules: paper, paint, metal, fabric, etc. The back of the card usually includes the artist's name, email, year the card was made, the number (if the card is part of a series), and possibly a title or theme.