That's an acronym that means Cascading Style Sheets. CSS allows web designers to attache styles (fonts, colors, size, margins, etc.) to HTML documents. It works like a template allowing web developers to define a style for an HTML element and then apply it to as many web pages as they want/need. We can also use styles to display different elements for different media (screen, printers). It simplifies web site maintenance and also helps the way a page displays content for Search Engine robots on the Internet (and that's a good thing!).