It's a store where you can buy various things with points (or in our case probably chips) that you earn for various things, most of which are already available by default here (bigger avatars, etc) so I'm not sure how many things would even be able to be in it.. but it's not an issue right now because it doesn't work with the version of the forum software we are running here.
The unfortunate thing about the VBPlaza or VBStore, whatever you want to call it--I usually just call it the "points system"--is that it isn't compatible with most versions of VBulletin. A forum I mod at had it a few times, but it was taken off for screwing up the forum/database (something technical I never bothered with finding out) and because people tended to abuse the system.
I think it's cool overall. I like getting rich, lol. But it's still mostly flawed when it comes to being compatible with newer versions of VB. I'm not an expert on VB or anything, I never did any coding, but that's what I've heard from my superiors on other forums when I asked them why the points system was taken away (on behalf of the members, since I honestly could've cared less.)