HTML is a markup language that defines the structure and layout of a web page. It uses tags like <p> and <div> to divide the page into headers, paragraphs, lists, and other sections. CSS and JavaScript can be used to style and add interactivity to HTML pages. Key HTML elements include headings, paragraphs, links, images, forms, tables, and lists. Attributes provide additional information about elements and usually appear in name/value pairs like name="value". Forms allow users to enter data using elements like text fields, textareas, select menus, radio buttons, and submit buttons.