HTML is a text-based markup language used for describing the structure of a web page, i.e. it is the publishing language for the World Wide Web.
HTML5, as a replacement of the current HTML4 and XHTML standards, is specifically designed to enable the presentation of rich content like vector graphics, animations, music and video, without the need for additional plugins.
For rapid development of websites, frameworks like Twitter Bootstrap can be helpful. They consist of HTML and CSS-based design templates and provide a common structure for responsive, mobile-friendly web pages.