With Global Play, players can create characters, set up friends lists, gather items, and use the gold-based auction house in every region where Diablo III is available. To access another region, simply click on the "Options" button from the login screen, select the "Account" tab, and then choose which region you want to play in from the "Regions" drop-down.
There are a few restrictions to keep in mind when using Global Play:
Using Global Play After a Patch
Global Play can only be used to connect to other regions that are running the same version of Diablo III as your home region. As a result, you may be unable to log into other regions using Global Play for a short period of time after a patch or game update has been released. This is because different regions will often be patched on different days.
Additional note: Your patching date is determined by the region of the installer that you used and not by the Global Play region you are set to.
Global Play and the Real-Money Auction House
While players can use the gold auction house in all Global Play regions (the Americas, Europe, and Asia), the real-money auction house will only be available when playing in your "home region." This means that, for characters outside of your home region, items cannot be bought or sold in any real-money auction house. If your home region is the Americas, for example, you will not be able to access the real-money auction house on characters you create in Europe or Asia.
For more information on Global Play in Diablo III, see the FAQ.