Most important thing about HTML 5 is that its now no longer part of SGML but its a language of its own. A number of new elements added as well as others removed or deprecated.
There are lots of new features that requires a detailed article but here I am trying to focus on major ones.
1. First of all simplified doctype for html 5 is:
2. For the purpose of drawing graphics, new canvas element has been added.
3. New Structural as well as Media elements has been added.
Structural -> Article, Command, Dialog, Summary, Progress etc.
Media -> Audio, Video, Track, Source etc.
4. A number of new form elements has been added as follows:
5. A number of existing elements has been deprecated or removed. Most important is Iframe removal.