CSS is an Abbreviation
It stands for Cascading Style Sheet.
Basically CSS is used design your web pages, for positioning of content and giving it a user friendly look and feel. Take a look at this page, the whole page is styled using CSS from the background colour to positioning of the right column and header text and menu at the top.
Without CSS styling this page would be black text on a white background and not very user friendly or pleasing to the eye.
Style sheet refers to the document itself. Style sheets have been used for document design for years. They are the technical specifications for a layout, whether print or online. Print designers use style sheets to insure that their designs are printed exactly to specifications. A style sheet for a Web page serves the same purpose, but with the added functionality of also telling the viewing engine (the Web browser) how to render the document being viewed.
Every Web page is affected by at least one style sheet, even if the Web designer doesn't apply any styles. This style sheet is the user agent style sheet - the default styles that the Web browser will use to display a page if no other instructions are provided. But if the designer provides other instructions, the browser needs to know which instructions have precedence.
CSS can save you a Lot of Work!
CSS defines HOW HTML elements are to be displayed.
Html elements are referred to usually as "Tags". You will learn in the following tutorials how to distinguish different tag elements.
Styles are normally saved in external .css files. External style sheets enable you to change the appearance and layout of all the pages in a Web site, just by editing one single file.