The difference between HTML and HTML5 is same as the difference between ASP.NET 1.1 and ASP.NET 4.0.
Well, concisely speaking, it's just an updated version of HTML, bundled with some new tags. The most important and perhaps most welcomed enhancement in HTML5 is better support for media (video and audio tags). It's the 5th major revision of HTML, so the name HTML5.
For more information about the difference between HTML (HTML4 actually) and HTML5, you can see the "W3C WORKING DRAFT" here:
For the list of new elements added with HTML5, refer to w3school:
Hope it'll help you.