It is not a totally new version of HTML 4 . It support the features of HTML 4 and is a collection of individual features like canvas, video, geolocation etc. We can interacts with javascript through DOM (Document Object Model). It does not have any special notation to handle features like video. The API defines this. Upgrading to HTML5 need only change the doctype.
!DOCTYPE html
It defines new semantics like article, section, footer etc.
Local Storage :
HTML5 provides local storage. It stores information on the mechine, this allows the sites to retrieve it later.
The Rails Drop 