Because different browsers support different versions of videos, you'll want to serve up more than one kind of video type to make sure people will be able to view your video. HTML5 is a helpful way to make sure you get the video playing on as many browsers as possible, because it also provides fallbacks. Here are several links covering this info that I've found to be very comprehensive and helpful:
http://www.sitepoint.com/the-princip...-online-video/
http://web.archive.org/web/201107250..._for_everybody
http://diveintohtml5.info/video.html