HTML is a markup language used to define the structure and layout of web pages. A basic HTML document includes opening and closing <html> tags which contain a <head> section for metadata and a <body> section for visible content. Common tags in the <body> include <h1>-<h6> heading tags, <p> paragraph tags, and <br> line break tags to structure text. HTML uses tags enclosed in angle brackets to mark elements like headings, paragraphs, and line breaks.