HTML5 is the latest version of Hypertext Markup Language (HTML), the code that describes web pages. It's actually three different kinds of code and each serves its purpose. HTML, which provides the structure; Cascading Style Sheets (CSS), which take care of presentation; and JavaScript, which makes things happen in the browser.
HTML5 was designed to accomplish almost everything you'd want to do online without requiring additional software or browser plugins. HTML5 does everything from animation to apps, music to movies, and can also be used to build incredibly complicated applications that run in your browser and on a variety of devices.