The term artistamp or artist's stamp refers to a postage stamp-like artform used to depict or commemorate any subject its creator chooses. Artistamps are a form of "Cinderella stamps" in that they are not valid for postage, but they differ from forgeries or bogus stamps in that typically the creator has no intent to fool postal authorities or stamp collectors.
If you are interested in participating in this project, you can read about it here. I was a bit surprised to receive the postcard and although I am having fun creating mail art, I've never done an artistamp before.