Everyone who has visited YouTube.com in the past four years knows that you can embed video in a web page. But prior to HTML5, there was no standards-based way to do this. Every video you've watched on the web has been funneled through a third party plugin (QuickTime, RealPlayer, Flash).
HTML5 defines a standard way to embed video in a web page, using a
<video width="300" height="150"></video>
Support for the <video width="300" height="150"> element is still evolving, which is a polite way of saying it doesn’t work yet. At least, it doesn’t work everywhere. But don’t despair! There are alternatives and fallbacks and options.
Video Codecs: There are tons of video codecs. The three most relevant codecs are H.264, Theora, and VP8.