Anything file format that iTunes and it's syncing engine support can be transferred both ways. That means music, video, iPhone apps, PIM data and email.

*But* there's no way to get iPhone apps other than the app store program on the iphone or via iTunes on the desktop. And the web browser can play MP3s/AACs on sites no problem, but there's no option to save them (therefore no way to transfer them from the iPhone to the desktop).