Firstly just an insight into the two types of streaming video that you will encounter on the internet.
Streaming Video is where the video buffers and starts playing before the full video is downloaded. You are able to move back or forwards to a certain point and watch it from there. Streaming media has additional advantages such as being able to broadcast live events, sometimes referred to as a webcast or netcast. In order to deliver true streaming video you will need a specialized streaming video host or purchase a license for Flash Media Server and have it running on your server.
Most videos that you look at on the internet day in and day out are progressive videos. Most viewers are not aware that they are watching progressive videos as they are just usually referred to as streaming as most people do not know any different. YouTube and other video sharing sites supply progressive Flash videos.
Progressive Download video is slightly different than true streaming and is where the actual video downloads on the viewer's computer, often into a temporary directory. The viewer can then watch the video when it downloads enough and there is something to watch. If the viewer's internet connection speed is fast enough and is downloading the video at a faster rate than the video will play the viewer can watch the entire video without stopping before the entire video is downloaded. If the viewer's connection speed is not fast enough then there will be a pause until more of the video downloads and it starts playing again. A progress bar shows how much has been downloaded. See the following video for an example.
From the above you may not be entirely clear on the difference between the two types but there is a big difference for the webmaster. The great advantage with using progressive video files, at least as far as the webmaster is concerned, is that they can be hosted anywhere and do not need a specialized hosting service and there are no additional license fees involved