The real difference is that media players can begin to show the video while it’s downloading. For example, a FLV file being delivered via HTTP Progressive download will begin to play in your Flash Player as soon as a little bit of data is received by the browser. The same is true for Windows Media files. Quick Time will wait until the entire file is downloaded before it plays, unless the QuickTime player on the PC/Mac is set for progressive play. So be careful when posting QuickTime videos.