An Excellent Article from Webmonkey: How HTML 5 Is Already Changing the Web.
Web has come a long way; from sites being just a collection of pages connected through hyperlinks, to supporting interactive applications and being powered by Flash, Silverlight and Ajax technology. Recently launched Google Chrome is an indication to where the web is headed. Browsers are going to be an operating system in their own right, handling complex web applications.
HTML 5 will further allow the developers to create pages taking advantage of the latest technologies in web development as well as support any browser on any platform. Have a look at the draft specification of HTML 5.