CSS3 Conditions?

I have heard with CSS3 there is a way to run some style properties when a condition is true. Does anybody know how to do that?

What type of condition?

Media queries allow you to specify conditionally based on some properties.
@supports rules will also be available very soon. Firefox / Opera / webkit have some support.

http://dev.w3.org/csswg/css3-conditional/
http://dev.opera.com/articles/view/native-css-feature-detection-via-the-supports-rule/