To link to other pages/sites. I've been looking at the HTML of the Yahoo group button I pasted into my weblog, and I can't quite make out what's going on (I have a vauge idea)
I assume you mean a HTML form button rather than a regular link, or an image that loosk like a button inside a link? You;d do that a bit like this:
<form>
<input type="button" onClick="document.location='http://www.whatever.com'">
</form>