CSS: Cascading Style Sheets
Why use CSS?
The basic premise for using CSS, is based on the idea that Web pages should separate content from appearance. The advantage of this approach is that updates and changes to the presentation need only be applied once (to the style sheet) to affect all pages; this also provides more flexibility and control of presentations characteristics, and can reduce complexity and repetition in the structural content. This allows especially large sites to achieve significant savings in page load times.
The WestCIV website provides a great one page note on why developers should use CSS.
CSS Zen Garden demonstrates the flexibility of CSS-based design.