If you played the already-released Diablo 3 game on PS3 or Xbox 360 then good news: you can transfer your characters' progress into the new game - Diablo 3: Ultimate Evil Edition - when it comes out later this month (19th).
You can also transfer your characters onto PlayStation 4 and Xbox One, regardless of whether you played the older game on PS3 or Xbox 360.
Do note, however, that you cannot transfer saves from PS3 to Xbox 360, or vice versa, nor can you transfer from PS4 to Xbox One or vice versa. You also cannot transfer from PC to console.
If you're playing the Ultimate Evil Edition on the same console as you did the standard Diablo 3 game then you need do nothing but play the game and it will automatically find your save file.
If you're upgrading from an older console to a newer console - of the same brand - there's another step involved. You need to run the old Diablo 3 game, update it to 1.0.3, sign into Sony Entertainment Network (the account you use to access the PlayStation Network) or Xbox Live and then go to the game's menu and select "More" and then "Export Save". Turn on your new console, sign into PSN/XBL, run the Ultimate Evil Edition of the game and your save should be found and imported.
If you're upgrading to a console of a different brand there's yet another step to go through: you must link your PSN/XBL accounts to your Battle.net account - which acts as a kind of intermediary, transferring the save-file from one company's network to another's.
Once you've done that - and there's a step-by-step guide if you need one - then it's just a case of running the old Diablo 3 game, patching, exporting the save and then playing the Ultimate Evil Edition. Hey presto, your save should be found automatically and imported.
If you've lost or traded your old Diablo 3 disc, or gotten rid of your old machine you played the game on, there's still a chance you can transfer your progress, providing you have access to your Diablo 3 save-file, be it in a cloud, on a USB stick or on an external HDD.
If you do have it, find a friend with the console you used to have, and a copy of Diablo 3, and patch the game and export the save-file you found to your XBL/PSN account. When you run the Ultimate Evil Edition on your new console at home - while signed in to XBL/SEN - your save will be found and your smile return.
There's extra step-by-step detail on Battle.net, as well as a FAQ should you have specific questions unanswered above.