Though many Diablo 3 fans have voiced extreme distaste for the real money auction house since it was announced, Blizzard is marching forward with their plans. They've set up a website to run players through the process, but here are the important bits:
For starters, players are only allowed to have ten auctions running at any given time. With any luck, this should cut down on item-farmers without impacting the average player too much. Auctions last 48 hours, and Blizzard takes a cut at the end of any successful sale.
Items are grouped into two categories which are treated a little different. Equipment sales are subject to a flat $1 fee on the Real-Money Auction House, and 15% of the sale on the Gold Auction House. Commodities (gems, materials, dyes, recipes etc) are subject to a flat 15% fee on both the Gold and Real-Money auction houses.
Further, if you want the sale of the auction to go to your Paypal account, it'll cost you a 15% transfer fee. No fees will be collected if you store the funds in your Battle.net account for purchasing other Blizzard games, subscriptions, or items on the Diablo III Auction House.
A 30% fee might seem like a hefty price, but well, it's 70% more than you've ever been able to sell game items for before Diablo 3. Right now, this seems like the best of both worlds. Blizzard gets a hefty cut for creating an awesome game, and top-level users gain an incentive to feed awesome gear down to their underlings.
You will also only be able to buy items off of your "home" region's auction house. Regions are currently, "Americas", "Europe", and "Asia" though Asia wont have a Real Money Auction House at launch.
That said, Global Play will allow you to play with friends in other regions. However, you wont be able to play any of your characters from other servers or buy things on their auction house. You can only create characters and play the game.