HTML5 is a cool new way of writing and using html mark-up. Previous versions of html (xhtml 1 & 2, html4) are quickly becoming the old way. While HTML5 is not fully accepted equally among all browsers it does appear to be the way to go. It's features add many conveniences which I will discuss here with you. When combined with CSS3, HTML5 can be quite awesome, indeed.
Previous doctype versions HTML4, xHTML2 as well as Dom Level 2 will sooner or later be replaced by HTML5. These versions are still currently valid and may always be, however, HTML5 neatly and conveniently covers all this terrain.
Here's just a quick breakdown of the nuts and bolts of HTML5. It's start's out with the doctype . The old doctype used versions strict or transitional. These included long links of which I'm sure most developers just copy and paste into the top of their html. I'll bet not one of us can actually remember how to write a strict doctype link from memory. The new doctype is now simply html. How's that for convenience. Also, the charset is simplified to utf-8. The only catch to the charset, besides the fact that it may not validate in html4 validators, is that is must be set within the first 500 bytes of the page.
This brings us to the point of validation. W3C has introduced a validator called UNICORN, which validates a page based on it's doctype. So if you're concerned that your page is not validating, (which I intially had issue with), not to fear. As HTML5 is emerging, it will begin to pick up more universal support from modern browsers. Thus, validators will have to re-issue releases of themselves that accept more and more HTML5. In essence, what does not validate today will soon come, in theory of course.