WordPress relies heavily on the presentation styles within CSS. With the use of Themes, you have an almost infinite choice of layout options. WordPress Themes make it easy to change your website’s appearance, and open up the field to help you create your own Theme and page layout.
CSS stands for Cascading Style Sheets. It allows you to store style presentation information (like colors and layout) separate from your HTML structure. This allows precision control of your website layout and makes your pages faster and easier to update.
This article briefly describes the use of CSS in WordPress, and lists some references for further information. For information on CSS itself, see Know Your Sources#CSS.