HTML is a markup language used to describe web pages. It uses tags to structure content into headings, paragraphs, lists, and other elements. Key HTML elements include <head> for page metadata, <body> for visible content, and <html> as the root element wrapping everything. Tags like <p>, <img>, and <a> are used to format text, embed images, and create hyperlinks. HTML documents are plain text that can be viewed and edited in any text editor.