Sure, xhtml is essentially html, but instead of using <BR> xhtml uses <br /> (Thats a line break by the way, in other words, a line down).
So xhtml is a more structured version of html and is used for outputting text and images to the browser etc.
CSS on the other hand works alongside xhtml/html to make it look nice, for example:
<p>Hello, World!</p>
This would just put Hello, World! on your screen, it will be in a horrible font and with no color, yuck.
If we add CSS though...
<style type="text/css">
p {
font-size:30px;
font-family:Arial, Helvetica, sans-serif;
color:#00FF00;
}
</style>
<p>Hello, World!</p>
Now its huge, in a nice font and in a lovely green color =D
Thats what CSS dose, it makes html look good.
And BC is blocked at my school =[
Hope this helps, Wolfy87.