Originally Posted by manyT
no guest pass mean, full game with limited time access
In the case of Diablo 3, the guest pass unlocks what Blizzard call a 'Starter Edition" in which you have unlimited playtime
however your character can not level higher than 13 and you can not get further than the Skeleton King chapter in the first act.
Once you reach these caps you are free to still play (Though there is nothing to really do), you can however start the game with another character (There are 5 classes).
When you upgrade to the full retail you resume where you where in the Starter Edition.