Do a search for "polyglot html" to find more information, and check out XHTML5 in a nutshell [blog.whatwg.org].
Yes, CSS3 works with XHTML5 (if your browser supports it), just as it would with HTML.
HTML5 added several new elements. A div (not new) is still a generic container. A canvas is used for drawing graphics (usually via script).
There's a good little primer called Dive into HTML5 [diveintohtml5.info] that has a lot of helpful info.