HTML is the standard markup language used to create web pages. It uses tags to define the structure of a document and includes elements like headings, paragraphs, images, and links. XHTML is a stricter version of HTML that requires elements to be in lowercase and closed properly. CSS is used with HTML to control how pages are displayed by defining styles separate from document structure and content. Basic HTML page structure includes tags for the <html>, <head>, <title>, and <body> elements.