HTML is a markup language used to define the structure and layout of web pages. It uses tags to annotate text with information about headings, paragraphs, lists, and other elements. The key HTML tags are: <html> to define an HTML document, <head> for metadata, <title> for the page title, <body> to contain the visible page content. HTML allows adding images, links, tables, and forms to pages and is essential for building web pages but has limitations as a presentation language.