CSS Style Sheets and more?

Hello,

First of all, what are CSS Style Sheets? How do they work? I use Dreamweaver by the way, and I have only made sites using html.

Also, I have seen a ton of neat sites made with CSS, and example being
http://mimi.captive-soul.com/bagginses/
I THINK it was made with CSS
My question is, where it says Welcome, Stats, Disclaimer, and so on, how are the neat lines under the words made? I think it looks so neat and organized that way, instead of just writing the word and underlining it. Anyone know how to do that? I'm trying to learn how to do stuff like that in CSS, but is there also a way to do that in HTML?

Thanks!

I think you can do in with html
basicly you would have to creat a table and use attribuates to move the text down and to the left (I know there are ones but I can't remember them I just use css for stuff liek that now, and then creat a 1 pixel border on just teh bottem of the cell (which is easy in css, dont know abotu html again)
as fro how css works thing of compressing all the atributes in one document into a single file that can be shared across several pages. I suggest you get a book if you want to learn it. It makes html must cleaner to read and is easyer then normal html atributes