CSS(Cascade Styling Sheet) is a file that "tells" the browser what styling to use for a page such as what font to use, what size the paragraph font is, the background color of the page, etc.
Usually, a CSS page has a name something like style.css (you use the .css extension) and it is stored on your server and called to in the page html.
When you use a stylesheet, you will have to mention it in the <head>'ing of the page you want to apply this style to. This also works with <style> tags.
<html>
<head>
<style>
BODY {BACKGROUND-COLOR:#000; COLOR:#EEE;}
</style>
</head>
<body>
<center><br /> This is white text on a black background </center>
</body>
</html>
