Cascading Style Sheets, or CSS, is a language used to specify the visual appearance of a Web page—in contrast to HTML (HyperText Markup Language), which is a markup language that defines the structure of a document for distribution on the Web. HTML tells a Web browser how the content is organized on the page, whereas CSS tells the browser how it should look.
CSS3, an abbreviation for CSS Level 3, is the next generation of this style language that adds several new capabilities. It may still be under development by the W3C (World Wide Web Consortium), but CSS3 has already taken its place alongside HTML5 at the forefront of all cutting-edge Web design.
Unlike CSS1 and 2, a single, comprehensive CSS3 does not exist. Instead, rather than trying to release the entire specification at once, the CSS Working Group has split the spec into a series of modules; each of which has its own developmental timeline.