HTML 5 is a change to the HTML specification (previously… HTML 4). It adds new elements (tags), removes old ones and clarifies some technical overheads to simplify matters for developers and browser vendors. The new elements allow you to present pieces of the document as having a more specific allocation than just placement in the page. Aside, Header, Article, Section, Nav etc.
The changes all provide more semantic meaning to the content of a page when the code is viewed by a computer, making it easier to reference areas as well as identify what the code is used for and even easier for machines (like search engines) to infer relevance and priority. Bottom line, you don’t really have to change much to move to HTML 5 since you probably weren’t using the obsoleted elements anyway.
HTML5! – the exciting web phenomenon – is really an attempt of the browser vendors (and W3C) at making all the features of desktop apps, Flash – and anything that previously needed a browser plugin – an integral part of the browser standard itself. The browser has become a sandbox for application development, whereas it was previously a platform for installing application development tools; something which often broke, angered/confused or opened security holes.
It’s worth noting that HTML 5 (the updated specification) hasn’t formally been agreed yet. That hasn’t stopped progress, however, the de facto ‘standards’ in HTML, CSS and JS are being rolled out continually, thanks to the new generation of ‘evergreen’ browsers (e.g. Chrome/Safari/Opera, FireFox and IE11). The timing of HTML, CSS and JS progress is somewhat coincidental (driven by demand), but it’s a great relief to developers that these things are coming together.
The web is the new host platform and HTML5! is the new Operating System.