HTML is a markup language used to define the structure and layout of web pages. HTML documents contain HTML elements that define different parts of the page like headings, paragraphs, lists, links, and more. Key HTML elements include <html> <head> <body> <h1>-<h6> for headings, <p> for paragraphs, <ul> and <ol> for unordered and ordered lists, <a> for links, <img> for images, <table> for tables, and <form> for forms. HTML documents are text files that use tags enclosed in < > to define elements and attributes provide additional information about elements.