HTML (HyperText Markup Language) is the standard markup language for creating web pages. HTML5 is the latest major revision of HTML, and it includes a number of new features and improvements over previous versions. HTML files contain text and tags that define the structure and content of a webpage, including headings, paragraphs, lists, images, videos, and links. Web browsers interpret these tags to display the webpage to the user. HTML5 introduced new semantic elements like `<article>, <aside>, <nav>, <header>, <footer>, and <section> to better define the structure of a web page. It also includes support for multimedia content through the <audio> and <video>` tags, eliminating the need for third-party plugins like Flash in many cases. HTML5 also provides APIs for offline storage, geolocation, and drag-and-drop functionality, enabling richer and more interactive web applications. HTML files are essential for creating websites and web applications, and they are supported by all major web browsers.