(Just in case you're on HS hiatus or totally ignore their news section)
Basically, they're going to split it up into Wild and Standard, and retire some stuff this spring.
Wild will be what we know now, where you build decks with whatever cards you have. Standard will be where you make decks with the current and previous year's cards.
I can't for anything figure out why they didn't call what we do now Standard though. Sounds like it'd be confusing.
BUT WE CAN BUILD 18 DECKS INSTEAD OF 9 WOOO
Then there's this thing...
Adventures and Expansions that are not part of the Standard format will no longer be available for purchase from the Shop—this year, that includes Naxxramas and Goblins vs Gnomes. If you want any cards you missed out on for Wild play or just to fill out your collection, you’ll be able to craft them using Arcane Dust—even cards from Adventures that were previously un-craftable. Speaking of Adventures, if you’ve purchased at least the first wing of an Adventure before it cycled out, you’ll still be able to finish acquiring and playing the remaining wings.