Cascading Style Sheets (CSS) is how modern websites create the style and layout of the site. A good web designer will seperate the content (the HTML) from the style (the CSS) – needless to say, we do.
Seperating content and style becomes very important for websites using Content Management Systems – a single CSS file can be edited and instantly applied across the entire website.
To get a better idea of how it works, take a look at this page without the CSS. Ugly, isn’t it!