HTML: HTML (HyperText Markup Language) is the basic markup language that describes Web pages. It’s a relatively simple technique for building Web sites that requires nothing but a plain text editor.
XHTML: XHTML is often considered the successor to HTML. Because it doesn’t allow certain kinds of tags, XHTML is actually a smaller language that’s a bit easier to use. Typically XHTML pages are more dependent on CSS than HTML, as many of the HTML tags are replaced with CSS tools.
CSS: CSS (Cascading Style Sheets) is a way to add specific style information to an HTML or XHTML page. HTML and XHTML provide the general framework, and CSS describes the color and layout.