Twitter have added iTunes App Store previews to their integrated services. Previously, Twitter had added Apple's Ping service to their integrated services, to allow you to preview songs before opening iTunes. It seems that Twitter have now added App Store integration as well. When someone tweets an App Store link, you will get a similar preview pane, which will show you, what app the link goes to, the price, a description and screenshots
This will only work on the new twitter website, won't work on the old website or on the official apps. Interestingly this new App Store integration only works with the iOS App Store and doesn't show any previews for Mac App Store Links right now.