We wrote back in September that Blizzard announced it would be removing Diablo 3's auction houses, both gold and real money, and that day has come to pass. The Auction House is no more.
Blizzard has posed a FAQ on its forums to answer some questions about the shutdown specifics. Auctions currently in progress will expire as normal and the items will go to the highest bidder (or be returned to the seller, depending on the auction). Folks have until June 24 to clear out all of the items and gold in their "Completed" tab, after which time they'll all vanish forever.
On paper, the Auction House sounded like a great idea: it provided a relatively secure way for Diablo 3 players to dispose of unwanted loot and make some coin at the same time—much more coin than the tiny amounts offered by the in-game merchants. However, the Auction House quickly became a required meta-game that any character wanting any kind of meaningful gear had to play. Blizzard had to adjust the rates at which valuable items spawned in-game to keep item-farmers from flooding the Auction House. This made it more difficult not just for the farmers to find good loot, but for everyone to find good loot. The end result was that the Auction House became the only way to find truly useful items for every character once they'd hit the level 60 in-game cap.
The removal of the Auction House isn't the only big change to Diablo 3's gameplay. In preparation for the Reaper of Souls expansion pack's upcoming debut (next week, in fact!), Blizzard also released "Loot 2.0" in February, a major patch that significantly changed how the game's loot system functions. With the auction house sunsetting, Loot 2.0 greatly increases the rate at which powerful items are generated for the player to pick up (orange legendaries and green set items). It also makes it so that items the player finds are much more likely to be relevant to the player's current character—your Demon Hunter might still find the occasional Witch Doctor charm, but for the most part, the items will have stats specifically targeted at your class.