Cascading Style Sheets (CSS) is basically a means to giving you greater control over the layout of the web pages that you create than traditional HTML does. Traditional HTML has its limitations that a designer has to take into account when creating layouts. With Cascading Style Sheets, you can define the look of any HTML tag, as well as positioning elements exactly where you want them on the page.
Anyone who is used to creating layouts with desktop publishing packages will quite likely become very frustrated when trying to design a web site. A designer can usually get around a design problem through the use of graphics. This isn't always the best solution though, as graphics increase download times.
Through the use of an external style sheet, you can drastically reduce the download time of your web site. Each page on your site has the minimum amount of code because they can all reference a single page that defines the style. These styles can take the place of graphics and tables that may have otherwise been used using traditional HTML.