HTML is a markup language used to structure and present content on the web. It uses tags to define headings, paragraphs, lists, and other elements. Common tags include <h1> for main headings, <p> for paragraphs, <ul> for unordered lists, <ol> for ordered lists, and <table> for tables. HTML pages have a basic structure with <html>, <head>, and <body> tags, and Cascading Style Sheets (CSS) can be used to style and lay out HTML elements.