DeSEO
Cascading Style Sheets
CSS are the solution to making HTML code more easily read, adjusted and maintained. A style sheet is used to create all the descisions about how the page appears - the style, which leaves the HTML free to do the content - the what.
This means that by altering only the CSS the whole look and feel, the branding, of a web-site can be changed, while leving the content intact.
The reason they are "cascading" is an indication of the manner in which they work, identical instructions at different points in the web page can produce different effects, depending on the what the CSS determines.