That's not strictly correct. Yes, a file is usually buffered and/or cached to a temporary location. But the entire file is not downloaded before opening the file unless the user chooses to do this.

Windows Media Player, RealPlayer, QuickTime, xine-plugin, mplayer-plugin - that's just a short list of multimedia browser plugins capable of playing media shortly after the download starts (not after it ends).

There are three major differences between using a so called "streaming" method and a primarily static file delivery system (such as HTTP or FTP).

1. In order to get an acceptable quality with streaming content, you need a sufficiently fast Internet connection. This is unique to streamed content.
   
2. Streams often support more efficient seeking than the alternatives.
   
3. Streams usually do not support useful caching. Ever major web browser does.

A fourth important concern is that no shared web host will stream your content for you. If you do get a shell account ($$$) so you can run a streaming server, you'll probably have to pay a large licensing fee ($$$).


The "Progressive Downloads" thing does not truly explain what happens or its support as I have just done. Basically, the users won't get to choose between saving the complete file or using a "progressive download" unless they know what they are doing.