At BlizzCon 2013 this past weekend, the Hearthstone
developers announced a new way to customize your decks: card backs, also called "card sleeves" among the TCG/CCG community. At the top of this post is one example, a Mists of Pandaria
-themed card back. The specifics of how these card skins will be unlocked is still up in the air, but the current idea is that you will earn the art by playing/winning X
number of matches per month.
Once you earn the card skin, you'll have it permanently. New card backs will be released on a regular basis. When creating or editing a deck, you'll be able to choose which skin you want to use for that deck. Both you and your opponent will see the skin you use during gameplay, giving the playing field a more diverse, customized appearance.