HTML (Hyper Text Markup Language) is the standard markup language for creating web pages and web applications. It is used to describe the structure of web content, using a process called tagging to define different elements such as headings, paragraphs, links, images, etc. These tags are interpreted by web browsers to display the content in a particular format. HTML is the backbone of any website and is commonly used in combination with CSS (Cascading Style Sheets) and JavaScript to create interactive and visually appealing websites.