HTML 5 is the latest version of HTML.
The biggest advantage of HTML5 is that it is totally free to everyone. Hence no need to buy or pay subscriptions for proprietary software and plugins. Even better it's cross-platform compatible, which means you can use a smartphone, a tablet, PC, or laptop to access the exact same web page. Eventually, from a developer standpoint, there will be no need to develop code based upon the device being used.
As with all major changes in software, language, or code there are always conflicts and complications. Video seems to have fallen with the "non-conforming" standards among developers. Although HTML5 supports most video, there has not been a consensus on what to support. Each browser or even device may have a different opinion on what to support. Ogg Theora, is supported by every browser excluding Internet Explorer while Safari support requires a manual installation. H.264, is supported by everyone excluding Firefox. VP8/WebM, is supported by all although Safari and IE require manual installations. Google announced that it would eventually remove H.264 support from Chrome. We are all waiting for the dust to settle.