http://us.battle.net/support/en/article/diablo-iii-starter-edition-guest-pass
"Considering that many gamers have waited almost a decade for Diablo 3, it is safe to assume that many have already pre-purchased the game or are planning to attend the launch events. Either way if you’ve got a friend who’s still on the fence or has no clue as to why the game is such a big deal, Blizzard has officially announced the Diablo 3 Starter Edition & Guest Pass. The Starter Edition will allow players to play until a level cap of 13 in the game and will make it all the way until the Skeleton King quest in Act I. They will not be able to access the Real Money Auction House either. The Guest Pass will essentially give players the same thing, but the Guest Pass can only be provided by those who purchased the box copy of Diablo 3 (much like the World of Warcraft guest passes). However if you weren’t invited to downloaded the game via the Guest Pass, the Starter Edition will only be made available for download 30 days (June 15 2012) after the game launches"