From Diablo Wiki
Patch Test Realm (PTR) is a server that Blizzard allow players access to in order to test future patches. They are online temporarily and shut down when the patch being tested has gone live on the regular servers.
The Diablo 3 PTR first went online in September 2012 in order to test Patch 1.0.5.
Unusually, for a developer, Blizzard also put unreleased games on the PTR. This may be to test pubic opinion on unreleased content not yet available for public testing without having to officially announce/confirm new additions and updates. The most recent titles to be released this way were Hearthstone and Reaper of Souls.
How it Works
To participate in the public test, players must have a Diablo 3 game license attached to their Battle.net account that’s in good standing (i.e. it hasn’t been suspended or banned). While the PTR will only be in available in English, French, German, and Korean languages, accounts from all regions are eligible to participate.
To access the PTR log into Battle.Net Europe or US. Select Diablo 3 from the list of games and download the PTR client. Once installed players can log in with their normal email/password/authenticator.
If it's the first time logging into the PTR players can create a new character here. Characters from an existing Diablo 3 account can be copied over.
How it Doesn't Work
Only characters from one region can be copied from a regular account onto the PTR. Players can transfer characters from another region but any characters on the PTR from a different region will be overwritten.
Characters from the PTR can not be transferred back to a regular account.