AVI and QuickTime are the most common downloadable video formats. AVI and QuickTime files can be previewed while they are downloading, but they don't truly stream.
Users have to wait for the file to download before viewing it.
The upside is that you can achieve higher quality with AVI and QuickTime clips, and they don't require special software to create, serve, or view. Here at CNET, we usually present downloadable files for content less than 30 seconds long. We use streaming video for longer clips. But streaming video is generally lower-quality than downloadable clips. To produce a streaming video, you have to download the encoder, and possibly pay a fee.