Some people are confused by CSS Specificity, especially with all of the (not-so) new CSS3 Selectors. The image below may help make sense of CSS Specificity.
X-0-0: The number of ID selectors, represented by Sharks
0-Y-0: The number of class selectors, attributes selectors, and pseudo-classes, represented by Fish
0-0-Z: The number of type selectors and pseudo-elements, represented by Plankton a la Spongebob
*: The universal selector has no value
Download the specificity chart and read the rest of the SpeciFISHity article here