HTML5 is a markup language used to structure and present content on the web. It uses tags to define elements like headings, paragraphs, and links. Some key tags include <html> and </html> to define an HTML document, <body> and </body> to contain the visible page content, and <a> to create hyperlinks to other web pages. Tags can have opening and closing parts or be self-closing like <br/> and <hr/>.