what’s different about html 5 ?

Html 5 is quite a large step in the evolution of the html standard. Support has been provided for features that figure in most modern web applications.

The guide by Mark Pilgrim is excellent. This is a summary of what I learnt from there.

HTML5 is fully backward compliant to HTML4 , meaning a browser which supports  html5 will continue to support your html4 content.

New features:

Canvas
A canvas is a rectangle in your page where you can use JavaScript to draw anything you want. HTML5 defines a set of functions (“the canvas API”) for drawing shapes, defining paths, creating gradients, and applying transformations.
Video
HTML5 defines a new element called <video> for embedding video in your web pages. Embedding video used to be impossible without third-party plugins such as Apple QuickTime or Adobe Flash.
Local Storage
HTML5 storage provides a way for web sites to store information on your computer and retrieve it later. The concept is similar to cookies, but it’s designed for larger quantities of information.
Web workers

Web Workers provide a standard way for browsers to run JavaScript in the background. With web workers, you can spawn multiple “threads” that all run at the same time, more or less.

Offline Web Applications

Offline web applications start out as online web applications. The first time you visit an offline-enabled web site, the web server tells your browser which files it needs in order to work offline. These files can be anything — HTML, JavaScript, images, even videos. Once your browser downloads all the necessary files, you can revisit the web site even if you’re not connected to the Internet. Your browser will notice that you’re offline and use the files it has already downloaded. When you get back online, any changes you’ve made can be uploaded to the remote web server.

Geolocation

Geolocation is the art of figuring out where you are in the world and (optionally) sharing that information with people you trust.

New Input Types
New Input Types in forms

  1. <input type=”search”> for search boxes
  2. <input type=”number”> for spinboxes
  3. <input type=”range”> for sliders
  4. <input type=”color”> for color pickers
  5. <input type=”tel”> for telephone numbers
  6. <input type=”url”> for web addresses
  7. <input type=”email”> for email addresses
  8. <input type=”date”> for calendar date pickers
  9. <input type=”month”> for months
  10. <input type=”week”> for weeks
  11. <input type=”time”> for timestamps
  12. <input type=”datetime”> for precise, absolute date+time stamps
  13. <input type=”datetime-local”> for local dates and times
Microdata
Microdata is a standardized way to provide additional semantics in your web pages. For example, you can use microdata to mark up an “About Me” page. Browsers, browser extensions, and search engines can convert your HTML5microdata markup into a vCard, a standard format for sharing contact information. You can also define your own microdata vocabularies.

Lastly, Minor tweaks like placeholder text and form autofocus.