HTML5 is not a single specification, but rather a collection of new HTML elements, APIs, and other technologies that together form the next generation of the World Wide Web. It includes new elements like header, footer, nav and video/audio elements, as well as APIs for graphics (Canvas), offline web applications, drag and drop and more. The document outlines the history and development of HTML5, provides examples of new HTML5 elements and CSS3 features, and discusses browser support and the future of HTML5 beyond the current specifications.