Designing with CSS
Cascading Style Sheets (CSS) enable website designers to style and lay out HTML data on web pages without the use of messy <table> and <font> tags, and without the use of "hacks" such as invisible GIF files. CSS is the recommended way for web authors to achieve good typography - much better than doing everything with HTML tags. HTML (Hypertext Markup Language) is the coded format language used for creating hypertext documents on the World Wide Web. For more information on CSS go to: Official W3C CSS Specification.
CSS allows for the separation of content from presentation, so that
web designers can change the entire design of a site by modifying one
style sheet, rather than updating separate HTML documents that make up the site.
What does this CSS stuff have to do with you?
With CSS, the content of your web pages is displayed more quickly because the code that explains how the site will look to the viewer, is written and stored in another file, a style sheet, separate from the html code. This makes your pages leaner and meaner.
On the design side, CSS gives your website designer more control and more flexibility in the creation and development of your website. Design is more precise and less temperamental.
On the deployment side, if you have a change in font size or font typeface, on say, 40 or more pages, only one file needs to be manipulated to make the changes globally (throughout your site). This saves time and energy on the part of the website designer. It saves you time and money.
There are many issues, some good, some not-so-good, that come into play when using CSS, that are too extensive for the scope of this document. If you'd like to read more about CSS, try Google's definitions of CSS.