Depression Glass was the cheap, everyday glassware produced during the 1930's when buyers couldn't afford anything "good".  As these things often go, what was once considered low class junk is now valuable.  Suntans are the same way.  For a while a tan was low class because it meant the woman had to work in the fields.  Then it became upper class because it meant the woman didn't have to work and had time to lay out in the sun during the middle of the day.  I'm not sure quite what it means now.

Given that Ebay currently has thousands of active listings for depression glass it has certainly retained it's popularity.  If you are a novice collector or just trying to identify a piece you might want to start with a depression glass book.