CSS3 is the latest version for Cascade Style Sheets available for now. Chrome, Mozilla, Safari and Opera support it, whereas IE (versions 7 and 8, not sure about 9) doesn't. You can do all kinds of things and get very cool features from CSS3, like rounded edges and even animations. Although using CSS3 excessively may slow things down a lot, especially making scrolling down painful in some browsers.
Want to see it in action? Here are a few demos: http://www.css3.info/preview/
One of the coolest ones is the transition one => http://www.css3.info/preview/css3-transitions/
