HTML5 is a markup language that defines the structure and layout of web pages. It uses tags to denote headings, paragraphs, and other elements. A website is a collection of webpages that uses HTML5 tags like <header>, <nav>, <section>, and <article> to structure content and provide semantic meaning. This example code shows the basic structure of an HTML5 webpage with a <header> containing an <h1> title and paragraph, and an <article> with contact information.