I've never even seen any HTML5 stuff, is it like flash, but better, or are there some drastic differences?
not sure since I haven't seen anything either
The reason you "haven't seen anything" of it is because HTML5 applications are just normal web applications rather than a stupid plugin. Agar.io? HTML5. Google Docs? HTML5. Twitch? HTML5. Discord? HTML5.
This was inevitable, Adobe stopped supporting Flash a long time ago and Flash was basically just a stopgap for web technologies that didn't exist for browsers yet. But now everything that flash did can be done natively in browsers, so it's rightfully deprecated. The only thing that was kind of nice about flash was that you could 'package' everything about your application into a single file (the swf file) and people could download and distribute them at will. Modern web apps don't tend to have that sort of portability.