HTML is the authoring language used to create web pages. It provides tags that give a web browser instructions on how to structure page content and features. Some key HTML tags include <body>, <i>, <HTML>, <title>, and <mark>. HTML5 allows for cleaner code by eliminating most <div> tags and replacing them with semantic elements. It also enables temporary local storage of structured data. HTML5 provides smarter caching of files to allow pages to load correctly offline.