HTML is the language used to create websites. It provides a logical structure for content using markup tags. Documents must begin with <!DOCTYPE html> to indicate HTML and <html> tags to enclose the content. The <head> contains metadata not displayed, including the <title>. The visible content goes in the <body> between its opening and closing tags. Some elements like <br /> are self-closing single tags. HTML organizes elements in a nested parent-child relationship structure.