HTML5 is the latest version of HTML that aims to improve the language with new multimedia and web application features while keeping it easily readable by humans and consistently understood by computers. It is still a work in progress but supported by major browsers. Key features include new elements for media playback, local storage APIs, and form controls. HTML5 also introduces new APIs for graphics, offline apps, and other features. It allows for cross-platform programming and powers mobile apps on platforms like Tizen, Firefox OS, and Windows 8.