HTML, XHTMl and CSS?  

To put it simply... CSS is a style sheet that you save as a file in your root directory.

In it you can write as many codes as you like. Each code contain the properties of all the basic things on a webpage like fonts, colour, background, border, table, etc...

Its purpose is to remove all those codes which keep repeating themselves on your pages. It keeps your pages cleaner... make them load faster... and make them easier to read by search engine's robots.

Instead of writing the code for your font and colour... you simply link the text to your file.css.

Another great advantage is that you can change the whole appearance of your website by simply editing a few codes in your css file. You can change the whole website in five minutes...
And another great advantage is that you don't need to re-upload your whole website. You only need to re-upload the css file after making your changes.

So by far... its greatest advantage is to save you a lot of time and giving you complete control over your website. One tiny little file controls the appearance of the whole website.

I used to have a website with 800 pages and it took me weeks of hard work to change the 800 pages... one by one. Later with the css file... it took me five minutes.

XHTML document contains the information for your site
CSS contains the style/look of your site.

By separating them out you speed up download times, increase accessibility, improve search engine rankings and as mentioned before decrease maintenance time.

Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can be applied to any kind of XML document, including SVG and XUL.