Hypertext Markup Language, commonly known as HTML, is a standard language for creating websites and web applications. It describes the structure of web pages using markup and is the backbone of all web content. HTML tags, which are surrounded by angle brackets, specify different elements such as headings, paragraphs, links, images, etc., on a webpage. These elements can be used together to customize the layout and appearance of the content on a webpage. HTML is often used with Cascading Style Sheets (CSS) and JavaScript to design complete websites.