CSS stands for Cascading Style Sheets. With the help of these Cascading Style Sheets it is possible to add new looks and feel to your XHTML document, also the content can be organized in an efficient manner. By using these styles, any webpage design can be re-created to match the designers mock-up file. Only a small change in CSS code will be required for redesigning the style of a newly added page. It should also be noted that it's better to keep the XHTML code and the CSS code separately. Style and Structure should always be kept separate when programming in XHTML.
For a person that knows XHTML or has previously studied XHTML, will find it much easier to learn CSS. So if you don't know the basics of XHTML, it would be much better to learn them before starting with CSS.
The main advantage of separating content from style is that in the case of wanting to change the appearance of a single element on every page of your website all you have to do is change the property within the CSS file. But in the case of XHTML, you would be required to make that change on every single XHTML page. Hence it is more designer and developer friendly.
Another advantage of CSS is that the page loads fast as it utilizes the browser cache feature in the user's system. Also good styles and designs can be made in a more detailed manner. So with the help of a good knowledge in CSS, better unique designs can be created.