CSS3 is the latest standard for cascading style sheets (CSS). CSS3 introduces several new modules that expand the capabilities of CSS, including selectors, box model, backgrounds/borders, image values, text effects, transformations, animations, multiple column layout, and user interface. The document provides examples of CSS3 properties and modules, demonstrating borders, selectors, text effects, menus, and creating multiple columns. It concludes by thanking some websites for information on CSS3.