There is lot of difference between HTML 4 and HTML 5 but one of the most important difference is with the HTML tags.

Few of the tags which would be used widely are video / audio tags embed tag and for visual representations some tags like <article>, <section>, <header>, <nav> .

Some of the elements which are gone from HTML 5 onwards are frame and frameset.

For complete set and list see http://www.w3.org/TR/html5-diff/