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.
What RSS is, how it can be used, how it came to be, how to use RSS syntax, how to make your first RSS document, Overview of the RSS <channel> element, overview of the RSS <item> element, how to publish your RSS doc, how to read other sites' RSS docs.