When you redeem a “guest pass” (Do not ask for them here) for Diablo 3, your account is upgraded to have the Starter Edition of Diablo 3.
Having played through it, it allows you to level any characters you want to around level 10 (You may play up till the slaying of a certain boss in chapter I, after that it’s incredibly difficult to get XP)
As of right now, the only way to get a starter edition, is to have a guest pass.
HOWEVER – After a set amount of time (30 days from launch), anyone will be able to register for a starter edition of Diablo 3, and be able to play for free for as long as they want (With the restrictions of the Starter Edition of course)
You still can get a bit of gameplay out of the starter edition, a good few hours depending on how you play. (I lurked in every nook and cranny of the game so I got a bit more out of it)
I’m not sure what the character limit is, or if one is imposed by the Starter Edition. You are able to create a character of any class of your choosing.