A language that manages the design and presentation of web pages: color, look, feel, and so on. It works together with HTML, which handles the content of web pages. Think of HTML as the skeleton of your webpages, while CSS is the clothing.
With CSS, you can create rules to tell your website how you want it to display information, and you can keep the commands for the style stuff — fonts, colors, and so on — separate from the commands for the content. They’re called “cascading” because you can have multiple style sheets, with one style sheet inheriting properties (or “cascading”) from others.