What are style sheets?

Style sheets describe how documents are presented on screens, in print, or perhaps how they are pronounced. W3C has actively promoted the use of style sheets on the Web since the Consortium was founded in 1994. The Style Activity has produced several W3C Recommendations (CSS1, CSS2, XPath, XSLT). CSS especially is widely implemented in browsers.

By attaching style sheets to structured documents on the Web (e.g. HTML), authors and readers can influence the presentation of documents without sacrificing device-independence or adding new HTML tags.

The easiest way to start experimenting with style sheets is to find a browser that supports CSS.

Source : http://www.w3.org/Style/