HTML (Hypertext Markup Language) is the standard markup language used to create web pages. It uses tags to define headings, paragraphs, lists and other elements. Originally developed to share scientific information, HTML is now widely used to format web pages using tags like <html>, <head>, <body>, <h1>, <p>, <br>, and <img>. HTML documents have a basic structure including a <DOCTYPE> declaration, <html> and <body> tags which contain other tags to define and structure the content.