A cascading style sheet is a way to simplify your life (once you learn how to use them) and to make your web site look nice, professional and consistent. There are plenty of tutorials on the web and in books to help you with this feature, so I won't go into the details here. What I will do is point you to some of the better resources that are available so you can make use of this powerful feature.
I've found that the CSS editor in Microsoft Frontpage 2000 is great. It really simplifies using a feature which is not all that easy to understand, much yet use.
Why would you want to use cascading style sheets in your web site? It's very simple. If you define all of your styles in one or just a few style sheets, you can make site-side changes almost instantly. In addition, this will ensure that the same styles are used throughout your site. This makes your web site look extremely professional and well thought out.
My recommendation: use cascading style sheets, unless you want to allow for older browsers that do not support them.
Unless otherwise noted, all photos and text is Copyright © Richard G Lowe, Jr.