Hi Dheeraj, that's a nice Question you raised, the Demand in Web Designing and Development has been increasing more these days and also it's really important to know the major characteristics that distinguish HTML5 from HTML4.
Simplified and Clear Syntax
Accessing User Geographical location
Client Side storage
Client Server Communication
That's just a Brief differences you find in HTML 4.0 & HTML 5. Hopw it's clear, if not do visit Go4expert.com
I would like to add few more points to what Kumar has written, HTML5 introduces a number of APIs that help in creating Web applications. These can be used together with the new elements introduced for applications:
An API for playing of video and audio which can be used with the new video and audio elements.
An API that enables offline Web applications.
An API that allows a Web application to register itself for certain protocols or media types.
An editing API in combination with a new global contenteditable attribute.
A drag & drop API in combination with a draggable attribute.
An API that exposes the history and allows pages to add to it to prevent breaking the back button.
HTML5 has several goals which differentiate it from HTML4.
The primary one is consistent, defined error handling. As you know, HTML purposely supports 'tag soup', or the ability to write malformed code and have it corrected into a valid document. The problem is that the rules for doing this aren't written down anywhere. When a new browser vendor wants to enter the market, they just have to test malformed documents in various browsers (especially IE) and reverse-engineer their error handling. If they don't, then many pages won't display correctly (estimates place roughly 90% of pages on the net as being at least somewhat malformed).
So, HTML5 is attempting to discover and codify this error handling, so that browser developers can all standardize and greatly reduce the time and money required to display things consistently.