In Chrome, Safari, and IE9, you can use H.264 with a video tag.
In Firefox, Chrome (and Chromium) and Opera, you can use Ogg with a video tag.
Flash usually uses H.264 (as I understand it).
The video tag may also contain a source tag:
<video>
<source src="video.mp4" />
<source src="video.ogg" />
<object ... ></object>
Sorry, no video for you
</video>
, .mp4, .ogg, flash ( (IE6/7/8)), , .
, , H.264, Ogg ogg H.264 Flash.