If you want to change something you only change it in that stylesheet and the entire site will change with it. The only browsers that really support CSS is Netscape 4.x and Internet Explorer 4.x though they don't both support everything. Now let's see how too make a CSS style. You place this in the head tag of your page and all your H2 headings will be like that . That way you can set the a predefined style for every element on the page.
Lets take a new example.
You may have noticed that some places get their links without those annoying underlines under them and if you use ie4 you may also have seen those links that change color or font or anything when you mouseover them. This class when applied to an element will make the text all uppercase, indent it by 5px and have the font Verdana. There are many many many properties you can set with CSS and you should at least know about most of them, so visit the World Wide Web Consortium's CSS reference page. Unfortunately Netscape and Internet Explorer have two totally different ways of doing that. In ie4 you can reach almost ANY element on a page like that.