HTML 5 is the latest version of the HTML markup language. It aims to improve HTML with support for multimedia and make it more compatible with modern web applications. Some key features of HTML 5 include the <canvas> element for drawing graphics, the <video> and <audio> elements for embedding video and audio, and local storage for storing data on the user's device. The development of HTML 5 was started by the Web Hypertext Application Technology Working Group in 2004 to address limitations of prior versions.