The World Wide Web was invented by Tim Berners-Lee in 1989 - 1991 while a scientist at CERN, the European Particle Physics Laboratory in Geneva, Switzerland. He created the web so people could share their papers through a web or network of "hypertext" documents. The people were to be connected randomly rather than in a systematic way as was usually done in computer networks. He invented HTML as the language that would "mark up" or tell browsers how to display the pages.
As the web grew, additional features were added to HTML. Today, HTML has many elements that control both the structure and the appearance of web pages. This inclusion of elements for both structure and appearance created a serious problem for both web designers and users of the web. Consider the example of a person who is visually impaired and has problems seeing certain combinations of color. Designers of web pages usually design colors for persons who are not visually impaired, and some inpaired people can not see the pages very well. These people would like to change the colors in the pages. However, the HTML elements that specify color are usually scattered throughout the pages and can not be changed by visitors to the site. To solve this problem, the World Wide Web Consortium (W3C), the organization founded by Tim Berners-Lee to coordinate web development world wide, specified that a set of rules to control the appearance of web pages be used by web designers. Visitors to web sites could change the rules to fit their needs. These rules became known as Cascading Style Sheets or CSS.
By specifying that CSS be used to control the appearance of web pages, the W3C wanted the control of the appearance of pages to be separate from the control of the structure of pages. CSS rules would control the appearance, and HTML elements, modified into XHTML elements to allow for better use by future browsers, would control the structure of the pages. This separation of appearance and structure is one of the key reasons why CSS should be used in web page design.
Follow these links to learn how to use CSS.