Topic 1 - Content and Structure
Content is what you want to show on your web page such as text, images, videos, and files. The content is basically everything the user is going to see on the page. The XHTML is where you define the content of the page and where it's located on the page such as a header or footer. You define them by using tags, if you want to put text in the page you'd use the p tag, which is a paragraph tag. If you want to put an image on the page you would have to use the img tag, the XHTML defines the structure of the page and where the content lies on the page.
Topic 2 - Presentation
CSS or the Cascading Style Sheets is how the content on the page is going to look, like the page color, text color, text being bold or italics and many other options for the style. You can change the style of a certain element in the content by defining the tag in the style sheets and then define what you want changed in the tag. The web page is nothing without the CSS; this is how you present your content to the user.
Topic 3 - The Rules of XHTML
Correct XHTML markups will give you the best chances for your page to be used on multiple devices for years to come. If your markups are well formed and both your XHTML and CSS are valid then it should compile with the web standards. Well formed markups means that your tags in your XHTML are structured correctly.