The document discusses HTML5 semantic and non-semantic elements. It defines semantic elements as those with inherent meaning, like <form> and <table>, while non-semantic elements like <div> and <span> do not convey meaning. New HTML5 semantic elements are introduced, including <section> for sections, <article> for independent content, <header> and <footer> for introductory and footer content, and <nav> for navigation links. Semantic elements are important for search engines and accessibility by clearly defining the meaning of different parts of a web page.