HTML is a markup language used to create web pages. It uses tags to denote structural elements like headings, paragraphs, lists, and links. Tags are written within angle brackets and indicate how content should be displayed. Common tags include <head> for the document head, <title> for the page title, <body> for the main page content, and <p> for paragraphs. HTML documents are created in a text editor and saved with a .html or .htm file extension.