More interactive and more media inclusive version of HTML 4.
Which means, in the past with HTML4 if you wanted to watch a video. You needed to download and install a particular codec like Realvideo or Microsoft’s mediaplayer or Apple’s Quicktime or Adobe Flash and so on. And that’s just video, you still need additional addons and doohickeys to view animations and listen to sounds.
Now with the advent of HTML5, all that and then some are all included so one could start experiencing all that right away without all that additional downloading, installing troubleshooting then eventually throwing away your computer.
Basically the evolution of HTML4 into 5 is to plug in all those missing components to give the user a more seamless pleasant experience without all the hassels.
However as in any new technology, there will be growing pains. Because not all browsers supports every new features of HTML5 so there will be compatibility and experience lapses as you visit these new HTML5 sites.
And more troublesome is HTML5 on smartphones and tablets. With any new technology, the demand on the platform to deliver that new experiences requires hardware that’s more capable by being faster, more better in every way.
So HTML5 is great and it is undoubtedly coming but beforewarned that there will be incompatibilities and frustrations along with some really kewl things.