CSS or Cascading Style Sheets are the contibuting STYLE element to todays websites. In the past every place you wanted to use a different font, size, color... you had to add these attributes directly into the html of your page. What you ended up with was a page overun with style coding if you wanted more than a very basic looking page.
Cascading style sheets allow you to define different styles, for example the look of title text, link text... and then easily reuse the style over and over as many times as needed without cluttering up the code of your web pages.
Today CSS has evolved to the point where you can use DIV instuctions to position elements on your web page. What this means is that todays practice of laying out web pages with tables to posion various elements may be history in the not to distant future. In our mortgage site designs CSS is a big component and some DIVs are already used for positong elements like page headers and menus which are typically done in off page Java Scripts.
The next step of course will be new designs done competley without tables. The benefits from a search engine optimization point of view are clear, a more eficient lighter design which allows positioning of the most important part of the page - the content at the start of the code.
While CSS designed websites are clearly the future their use is still somewhat limited. Some major sites like ESPN and Adobe Studio are done in this new way and blogging software but the bulk of websites are still done with tables.
Blog softwares like WordPress and Movable Type use CSS. If you want to see another advantage of CSS just select the theme switcher on many a blog you find, what you'll see is a complete change in the look and feel of the blog while all that has changed is the CSS style sheet.
To read more about the eventual demise of tables design visit this link:
http://www.hotdesign.com/seybold/