Hardly a buzz-word, more the “hail-mary” of markup language progression, HTML5 brings very important tags and standards to the web.
First and foremost, HTML5 solves the question from that last couple of years of what DOCTYPE to use with XHTML.
John Resig (creator of jQuery) has the best write-up on HTML5 link to John Resig site.
HTML5 also brings three new “Flash killer” tags (when I write “Flash killer” I intend that a lot of frivolous reasons people use flash for, will be replaced by these tags, not flash will be gone).
The <video> tag (YouTube demo) will eventually replace Flash Video, and hopefully SilverLight too, because it offers no reliance on Flash and better compatibility with mobile devices of the future. As in it is easier to detect a video format to play based on browser than to have a flash video player mobile device. Also, flash video does have some basic limitation and does sometimes hog more CPU than required to show a video. Especially if you can have a more core level video decompression happening on a machine. One issue that has been popping-up is the age old, what video codec to support. My two cents say, doesn’t really matter, browsers should try to support as many as they can and a clear winner will prevail, hopefully one that doesn’t exist yet. Van Kesteren has a good write-up on the issue, he is a bit biased though.
<audio> has less of a struggle, because MP3 is supreme and it would be great to see Soundmanager get an upgrade in using this new tag. This would replace the enormous amount of small flash mp3 players that are scattered about the web.