The document defines HTML (Hypertext Markup Language) and basic HTML tags. HTML is a markup language that uses tags to define elements like headings, paragraphs, links etc. in a web page. HTML files have an .html or .htm extension and can be created with a basic text editor. Common HTML tags include <h1> for main headings, <p> for paragraphs, <a> for links, <img> for images, and <table> for tables.