You can even customize this behavior. By default, the indicator Ron mentioned shows the posts that have been updated since you last logged in.
If you go to my profile > site personalization, you can check "Keep track of threads read on a thread by thread basis. This shows the unread flag for all threads you haven't read since you clicked the link to mark all threads as read (or when you enabled the setting.) I like this feature because I can mark threads I want to go back to as unread.