Cascading Style Sheets are a tool which allows Web designers to make their websites more attractive. They allow designers to specify how they want the Internet browsers to present their sites.
HTML was never meant to be used to give browsers detailed instructions about the appearance of sites. It dates back to a simpler time when the Web was mostly used for exchanging academic information. Attractive presentation wasn't as important as it is today, when a website needs to try hard to get itself noticed. When that shift first became apparent people did try to use HTML in order to make their sites more interesting. They came up with some creative ideas but they lost the elegant simplicity and universality of well-crafted HTML. The Web needed a better way to let designers control the appearance of their sites. Cascading Style Sheets were the solution.
As Richard Butterworth said in his lecture on this topic a style sheet is simply "a set of instructions that identify the style to be used in publishing a manuscript". (Lecture 6: Markup and Cascading Style Sheets.) A Cascading Style Sheet is a set of instructions for the Internet browser. It tells the browser how to present a particular webpage or group of pages. In essence this is the same as the style sheets that publishers and newspapers use to specify how their documents should be printed.
The clever thing about CSS is that it is very versatile. For example it can be used to efficiently present a number of web pages in the same style. The information only needs to be read once.
On the other hand CSS can also be used to present the same web page in a variety of styles.
The Zen Garden website is a thought provoking showcase for the latter technique. Visitors can choose to see the site in a huge range of different styles. Each style created by a different designer, each one of them striving to demonstrate exactly how versatile Cascading Style Sheets really are.
I have used CSS to control the appearance of my website. I make no claim to be ready for Zen Garden, but it it is clearly a very useful tool.