Then in 2012 came another split of W3C and WHATWG, the WHATWG renamed HTML5 in just HTML and their goal is to consider HTML as a living standard that evolves continuously and doesn't need any version number, more about this topic can be found in Ian "Hixie" Hickson mail. This is how browser Vendors implement new features, they don't implement everything at once and then release a new version containing tons of new features. Mozilla, Google and Opera have chosen to implement new features as fast as possible and release a new browser version every few weeks. This makes it difficult for developers to keep up the pace, but also ensures that new features get rolled out as fast as possible an can be tested and used.
Initially the W3C had planned to release the final version of HTML5 around 2022. But they recently announced a new date which is 2014.
What is new in HTML5
On the other hand it's hard to deliver exact informations about what is HTML5 and what not because the W3C took stuff out of the HTML5 specification in the past to make separate specifications. Which again in my opinion is good because several small specifications can have their own timeline and don't need to follow the release dates that got set for HTML5.
Another great new thing for developers is that the HTML5 draft also describes how browsers should parse HTML5. It's not up to the vendors anymore to define how they want to parse HTML, it's now clearly defined in the specification. This means for us developers that an HTML5 websites that was built in one browser will behave and look exactly the same in all the other HTML5 ready browsers. Ok, ok, this sounds like a huge plus, because every web developer knows how much work you spend on every project to make the application behave the same in every browser. Is it really true that you will write an app once following the standards and then it will work without tweaks in every browser? Honestly i'm a little bit pessimistic, but we need to wait and so, hopefully it will be so in a not so far future.
The advantages of HTML5
In my opinion one big plus of HTML5 because of SEO performance, is the semantic plus HTML5 adds to your website, combined with microdata your website could increase it's ranking and visibility in search engines because of rich snippets.
Another advantage of HTML5 is that it can already be used today. Of course you may have to also implement fallback methods for older browsers, but the majority of browsers out there already support a lot of HTML5 features. Especially in the world of mobile web-apps a lot of HTML5 is used, phonegap is a very good web-app library that makes extensive use of HTML5. A good example are the semantic tags which start to appear in lots of websites. Other features like the new Video tag and API get tested on websites like Youtube and will be the default technology used sometimes soon.
Not everything is HTML is totally new, but now those things are standards
Standards are good. Standards make a developers life a lot easier. Without standards implementing a new feature in an application is tricky, take a lot of time and often needs a lot maintenance after you implemented it.
All that stuff already existed before, but the big advantage is that now all this is a standard and all browser will implement these features the same way. Right now if you want to use HTML5 you will need lots of quirks and hacks especially if you want that same functionality across browsers (especially IE) because you still have to support older browsers that are not HTML5 ready. But in the future when HTML5 will be finished and as soon as all browsers support HTML5 you will be able to use all these features without hacks and extra libraries.
Another advantage of HTML5 is, it now supports features like IRI's (Internationalized Resource Identifier). If your document uses UTF-8 / UTF-16 encoding, IRI's are like URI's, but while URI's may accept all ASCII characters, IRI's accept unicode characters depending of which encoding you use (UTF-8 / UTF-16).