Blizzard is overhauling some core features of Hearthstone, adding a new format to their addictive digital card game and, perhaps most importantly, giving players more deck slots. In short, Hearthstone multiplayer games will now be separated into two modes. There's "Standard" mode, which will require players to use decks built from cards released in the "current and previous calendar year" as well as Basic/Classic packs. Right now that list includes the expansions Blackrock Mountain, The Grand Tournament and League of Explorers.
There's also "Wild" mode, which is essentially the Hearthstone we have now: any card from any expansion pack.
You can read more about the changes over at battle.net. Blizzard's designers hope that this new Standard mode — which will be the default format for competitive Hearthstone play — will help ensure that the meta-game doesn't get too unbalanced and out of control.
Blizzard also says that before this Standard update goes live in autumn, they plan to double players' deck slots, finally addressing one of the game's most-requested features. When that buff hits, every player who has unlocked all nine heroes will be able to juggle between 18 decks.