What is HTML 5?

Everyone's using it, nobody knows what it is. I realize that sounds more like a line out of an existential movie -- maybe Waiting for Godot or a screenplay by Sartre -- than a statement about HTML5. But it's really the most of the people using HTML5 are treating it as HTML4+, or even worse, HTML4 (and some stuff they don't use). The result? A real delay in the paradigm shift that HTML5 is almost certain to bring. It's certainly not time to look away, because by the time you look back, you may have missed something really a subtle but important transition centered around HTML5.