Help a noob around :D

[Marthenil]

As the title suggests im new to the Diablo series in general (Don't ask, I REALLY up to this day don't know why i haven't played the rest).
I've played however both Torchlight and Titan Quest, which are Diablo clones, so combat etc is not something I'm not accustomed with.
So, to the question itself: What's with the quest selection when you are about to log in? What's the current progress that you lose if you select a quest phase you want to redo?
Thanks a lot in advance for your answers, and sorry if a same thread exists somewhere, I had a quick search and didn't find anything.