The two organizations currently responsible for the development of HTML have decided on a degree of separation and this means that in the future there will be two versions of HTML5 - the snapshot and the living standard.
The Web Hypertext Application Technology Working Group (WHATWG) was formed in response to the slow progress being made by the World Wide Web Consortium (W3C) on the future of HTML. In fact "W3C more or less gave up on HTML to concentrate on XML and XHMTL in particular. The problem at the time was that most users wanted to stay with HTML and simply wanted it to be improved.
As a result WHATWG as been responsible for pushing the HTML standard forward since 2004 and around 2007 the W3C adopted the WHATWG specification as HTML5. The two organizations have been nominally working together on HTML5 ever since - but that have been some big differences in approach.
Over the years the two organizations have tended to diverge even more in their approach and now the fork is being formalized.
In a post to the WHATWG list, Ian "Hixie" Hickson, the editor of the WHATWG specifications explains:
More recently, the goals of the W3C and the WHATWG on the HTML front have diverged a bit as well. The WHATWG effort is focused on developing the canonical description of HTML and related technologies, meaning fixing bugs as we find them adding new features as they become necessary and viable, and generally tracking implementations. The W3C effort, meanwhile, is now focused on creating a snapshot developed according to the venerable W3C process. This led to the chairs of the W3C HTML working group and myself deciding to split the work into two, with a different person responsible for editing the W3C HTML5, canvas, and microdata specifications than is editing the WHATWG specification.
In plain terms this means that the W3C will continue to work on the HTML5 specification. WHATWG on the other hand will continue its work with HTML5 as a "Living Standard". The idea of a living standard is that it never settles down and is always being added to and refined. The task of the browser makers and the programmers using HTML5 is to try to keep up.
If you think that these two organizations are now going their separate ways and that this means that there will be two HTML5 standards, I think you are likely to be correct. The W3C is planning to create a single definitive standard, which WHATWG regards as a "snapshot" of its living standard HTML5. Freed of the need to stay in step with the W3C, it is likely that the WHATWG's living standard will move ahead even faster than before.
Overall this doesn't seem to be a good development. It will no longer be possible to say exactly what HTML5 is - the W3C's snapshot or the living standard of WHATWG. My guess is that Chrome and Firefox will adopt as much of the new stuff as possible and Microsoft will simply pick which bits most suit its corporate interests.
So in the future you will need to choose what HTML5 means to you.