Hearthstone is once again changing "years," which means that old cards are going away and becoming "wild" (Whispers of the Old Gods, One Night in Karazhan, Mean Streets of Gadgetzan) and new cards are becoming "standard" (tournament ready). The Year of the Mammoth is ending and will morph into the Year of the Raven.
Alongside of the aforementioned new cards will come a new Druid hero (Lunara, which you can win after 10 successful games in the new standard format), and more tournament opportunities with bigger prize pools. One of the biggest additions is the enhanced Fireside Gathering feature, which will make it easier to run player-led tournaments within the Hearthstone client -- think of it like a tourney mode in a popular fighting game.
The helpful infographic below shows off all of Hearthstone's expansions since 2014, and codifies that the Year of the Raven will host three of its own, with single-player content confirmed for all of them. For reference for all you hardcore players out there, the Coldlight Oracle, Ice Block, and an un-nerfed Molten Giant are being retired to the hall of fame, all of which are also in the gallery below.
via destructoid https://www.destructoid.com/hearthstone-moves-into-the-year-of-the-raven-with-some-big-changes-491910.phtml