<video id="vid1" width="400" height="300" poster="screen.jpg" durationHint="20">
Gareth's answer is the ideal, BUT it will only work in browsers that support the
video element and, even then, only in browsers that support whatever format you used to encode your video. And there currently is no single format that is supported by all browsers (even all "modern" browsers).
To get it working cross-browser, you have to use multiple encodings and a technique like Video For Everybody.
This is very similar to the link Doug posted, but goes into a lot of detail (more than I could care for, but still extremely helpful).