This is called Cascading Style Sheets (CSS) and is part of DHTML. The 'a' section defines normal links. The 'text-decoration:' area defines whether or not they are underlined. I think the color and font-weight sections are pretty self explanatory. When someone scrolls over the link, if you want it to change colors, or be underlined then you would modify the 'a:hover' section. Modify the 'a:visited' section, for the links to look different after they have visited them. You can also have different links do different stuff, like your navigational links. Just specify a class. To do that, you just add the name for the class like so 'a.class' or 'a.class:hover' for hover and visited characteristics.
Make sure that the info in your body tag doesn't conflict, and put the style info in between the head tags in your document.
By the way, this just bruhes the surface of what you can do with DHTML. If you want to see this code in action, then visit this web site.