HTML is a markup language used to define the structure and layout of web pages. It uses tags to mark elements like headings, paragraphs, links, and images. Some key HTML tags are <h1> for main headings, <p> for paragraphs, <a> for links, <img> for images, and <form> for forms. The <html> tag forms the root of an HTML document, with <head> for metadata and <body> to contain the visible page content.