Style sheets are a very powerful tool for the Web site developer. They give you the chance to be completely consistent with the look and feel of your pages, while giving you much more control over the layout and design than straight HTML ever did.
What is CSS? - Cascading - What Does it Mean? (Page 4 of 5 )
Cascading is something that makes CSS even more powerful. Style sheets cascade when The Web Writer or user (or both) have created an order of precedence for the browser to apply the style rules in multiple sheets. The style rule or sheet that has the highest precedence is the one that is used. The following list is a simplification of how your browser decides precedence for a style:
Look for the style element that is created, if it is not in the document, use the default rules in the browser.
Determine if any of the style rules are marked as important and apply those to the appropriate elements.
Any style rules in the document will have precedence over the default browser settings.
The more specific the style rule, the higher the precedence it will have.
Finally, if two rules apply to the same element, the one that was loaded last will have the highest precedence.
Style Sheets 
/ 13 
HTML5 Boilerplate Introduction
