The cards are traded or exchanged with others via snail mail. The ATC movement developed out of the mail art movement and has its origins in Switzerland. There's a number of online groups where those who wish to swap cards can connect.
They are not typically created to be sold, but when they are, they would be referred to as Art Card Editions and Originals (ACEOs).
There are lots of online tutorials to help you get started. They are fun to make and fun to share. I'm addicted.
Here are some of my first card creations...
|Halloween Prototype For An Upcoming Swap|
|Prototype foe an Upcoming Owl Lovers Swap|