Thanks in large part to Steve Jobs, HTML5 has been getting a lot of attention over the past few months. HTML5 is an emerging standard that covers video and too, provides new tools that allow Web browsers to largely replicate functionality provided today by runtimes such as Adobe Flash and Microsoft Silverlight. Since HTML5 requires the developers of the major browsers, including Microsoft, Mozilla, Google, and Opera, to implement the standard,differences in implementation and incompatibilities can emerge. These differences also relate to how HTML5 video is supported by the various browsers. At present there is no single HTML5 compatible video format supported by all browsers on the market.
To learn more about HTML5, we suggest checking out http://diveintohtml5.org/, an excellent resource for developers and novices alike. For video-related topics, check out the subsection at http://diveintohtml5.org/video.html.