Cascading Style Sheets (CSS) are used to control and change the style and layout of web pages. You can create style sheets that define how different elements, such as headers and links, appear on your webpages. By changing the style sheet, you can quickly change the look and feel of all web pages that utilize the same style sheet.
Here is a link for more info from Wikipedia:
http://en.wikipedia.org/wiki/Cascading_Style_Sheets