The term represents two different concepts. It is a new version of the language HTML, with new elements, attributes, and behaviors, and a larger set of technologies that allows the building of more diverse and powerful Web sites and applications. This set is sometimes called HTML5 & friends and often shortened to just HTML5. Designed to be usable by all Open Web developers, this reference page links to numerous resources about HTML5 technologies, classified into several groups based on their function.
HTML5 Tutorial. How to mark up subheadings, subtitles, alternative titles and taglines. If you don’t already know, the hgroup element is obsolete in HTML5.