Pseudo classes are those things with colons in them, you know, like :hover or :link. They let you control the CSS of stuff in different forms, states and places easily and efficiently, without taking up too much room. CSS3 is going to introduce a ton of new pseudo classes, and they’re going to make our lives a lot easier if you take browser compatibility out of the equation. They help both aesthetics and usability, and can make things that were once hard easier than ever before…
Direct link: The Mysterious Pseudo Class in CSS