The front side depicts a piece of art in a horizontal orientation. There are no foil art cards. Instead, all of them are treated with a special developed gloss varnish that brings out the art. The card backs are mostly blank, with some collector's info (Magic logo, illustration credits, legal text and collector numbers). The backs are left uncoated. They are similar to artist's proofs and thus provide room to get an artist's signature or sketch on it. They can also be used as tokens for copy effects, proxies, or if it is for a legendary creature it could be a good way to store experience counters, life total, commander tax, etc.
Arts cards were first featured in the Modern Horizons Art Series. They haven't appeared in other sets yet.