Wikipedia has a nice intro: https://en.wikipedia.org/wiki/Comparison_of_HTML5_and_Flash
It’ll take a while, but even Adobe is feeling the heat. They recently decided to stop developing Flash for mobile (including Android) and their developer tools have rudimentary HTML5 support.
This is just a stupid game that the framework vendors like to play. Apparently nobody really likes standards unless it’s their own. Before Flash there was Java, after Flash and before HTML5 there was Silverlight.
IMO, though, it’s nice to see Flash losing ground to a more open-sourced solution, if only because Adobe solutions tend to be cesspools of bloat, bugs and vulnerabilities. They’re the Microsoft of the creative world, meaning they’re so entrenched in their position of power they stop innovating and really caring about their customers. Instead they sit on their lazy asses, fix things only when embarrassed by the press, and make minor changes to break compatibility with new versions just to milk another generation’s worth of income. It sure would be nice to see better, more open, editing suites. That, to me at least, is why HTML5 is a little exciting.