Dominaria United/Art series
The Dominaria United Art Series is a series of 81 art cards that can be found in Dominaria United Set Boosters.
The front side depicts a piece of art from the set in landscape orientation (except for the planeswalker cards, which are displayed in portrait). The card back is mostly blank, with some collector's info (Magic logo, illustration credits, legal text and collector numbers). They are similar to an artist's proofs and thus provide room to get an artist's signature or drawing on it. They can also be used as tokens for copy effects, as proxies, or if it is for a legendary creature it could be a good way to store experience counters, life total, commander tax, etc.
There are no foil art cards. Instead, all of them are treated with a specially developed gloss varnish that brings out the art. The backs are left uncoated. Five percent of the time, rather than a normal art card, you'll get one with a gold-stamped signature of the artist. Every art card exists in both its normal version and its signature version.