CSS helps to keep the information content of a document separate from the details of how to display it. The details of how to display the document are known as its style. You keep the style separate from the content so that you can:
Make maintenance easier
Use the same content with different styles for different purposes
Your web site might have thousands of pages that look similar. Using CSS, you store the style information in common files that all the pages share.
When a user displays a web page, the user's browser loads the style information along with the content of the page.
When a user prints a web page, you provide different style information that makes the printed page easy to read.