HTML is a markup language used to define the structure and layout of web pages. HTML uses tags to mark elements like headings, paragraphs, lists, links etc. Some key HTML tags are <h1> for main headings, <p> for paragraphs, <ul> for unordered lists, <ol> for ordered lists, <a> for links, <img> for images. HTML documents have a basic structure with <html>, <head> and <body> tags. CSS can be used to style HTML elements by using the style attribute or external style sheets. HTML also supports basic text formatting tags like <b>, <i>, <u>. Colors in HTML are specified using hexadecimal color codes.