HTML is the lingua franca for publishing on the World Wide Web. XHTML is the latest version of HTML. To learn more, read W3Schools' HTML and XHTML tutorials.
After you have studied our W3C tutorial, you will know what W3C is. You will know about the World Wide Web Consortium, and how the Web is standardized.
The Semantic Web = a Web with a meaning. "If HTML and the Web made all the online documents look like one huge book, RDF, schema, and inference languages will make all the data in the world look like one huge database" --Tim Berners-Lee, Weaving the We