HTML5 is a language for structuring and presenting content for the World Wide Web, a core technology of the Internet. It is the latest revision of the HTML standard (originally created in 1990) and currently remains under development. Its core aims have been to improve the language with support for the latest multimedia while keeping it easily readable by humans and consistently understood by computers and devices (web browsers, parsers etc.). It adds a great Multimedia and Semantic Web capability to the internet!
 
Following its immediate predecessors HTML 4.01 and XHTML 1.1, HTML5 is a response to the observation that the HTML and XHTML in common use on the World Wide Web is a mixture of features introduced by various specifications, along with those introduced by software products such as web browsers, those established by common practice, and the many syntax errors in existing web documents. It is also an attempt to define a single markup language that can be written in either HTML or XHTML syntax. It includes detailed processing models to encourage more interoperable implementations; it extends, improves and rationalizes the markup available for documents, and introduces markup and APIs for complex web applications.
 
In particular, HTML5 adds many new syntactical features. These include the <video>, <audio>, and <canvas> elements, as well as the integration of SVG content. These features are designed to make it easy to include and handle multimedia and graphical content on the web without having to resort to proprietary plugins and APIs. Other new elements, such as <section>, <article>, <header>, and <nav>, are designed to enrich the semantic content of documents. New attributes have been introduced for the same purpose, while some elements and attributes have been removed. Some elements, such as <a>, <cite> and <menu> have been changed, redefined or standardized. The APIs and DOM are no longer afterthoughts, but are fundamental parts of the HTML5 specification. HTML5 also defines in some detail the required processing for invalid documents, so that syntax errors will be treated uniformly by all conforming browsers and other user agents.
 
Apple Inc’s CEO Steve Jobs issued a public letter titled “Thoughts on Flash” where he concludes that with the development of HTML5, Adobe Flash is no longer necessary to watch video or consume any kind of web content. This sparked a debate in web development circles where some suggested that while HTML5 provides enhanced functionality developers must consider the varying browser support of the different parts of the standard as well as other functionality differences between HTML5 and Flash.
 

Markup

HTML5 introduces a number of new elements and attributes that reflect typical usage on modern websites. Some of them are semantic replacements for common uses of generic block (<div>) and inline (<span>) elements, for example <nav> (website navigation block), <footer> (usually referring to bottom of web page or to last lines of HTML code), or <audio> and <video> instead of <object>. Some deprecated elements from HTML 4.01 have been dropped, including purely presentational elements such as <font> and <center>, whose effects are achieved using Cascading Style Sheets. There is also a renewed emphasis on the importance of DOM scripting (e.g., JavaScript) in Web behavior.
The HTML5 syntax is no longer based on SGML despite the similarity of its markup. It has, however, been designed to be backward compatible with common parsing of older versions of HTML. It comes with a new introductory line that looks like an SGML document type declaration, <!DOCTYPE html>, which triggers the standards-compliant rendering mode. As of 5 January 2009, HTML5 also includes Web Forms 2.0, a previously separate WHATWG specification.
 

New APIs

In addition to specifying markup, HTML5 specifies scripting application programming interfaces (APIs). Existing document object model (DOM) interfaces are extended and de facto features documented. There are also new APIs, such as:

  • The canvas element for immediate mode 2D drawing.
  • Offline storage database (offline web applications)
  • Document editing
  • Drag-and-drop
  • Cross-document messaging
  • Browser history management
  • MIME type and protocol handler registration
  • Microdata
  • Geolocation
  • Web SQL Database, a local SQL Database.
  • The Indexed Database API, an indexed hierarchical key-value store (formerly WebSimpleDB)
  • File API, Handle file uploads and file manipulation.
  • Directories and System. This API is intended to satisfy client-side-storage use cases not well served by databases.
  • File Writer. An API for writing to files from web applications.

A common misconception is that HTML5 can provide animation within web pages, which is untrue. Either JavaScript or CSS3 is necessary for animating HTML elements. Animation is also possible using JavaScript and HTML 4.
 
Sources: http://en.wikipedia.org/wiki/HTML5