CSS means Cascading Style Sheets. This is a stylesheet language and it is generally used from web designers to describe the presentation of a web page. CSS can be used in HTML and XHTML as well as XML documents. The CSS specifications are maintained by the World Wide Web Consortium (W3C).
Cascading Style Sheets have different levels and profiles. According to wikipedia, “each level of CSS builds upon the last, typically adding new features and are typically denoted as CSS1, CSS2, and CSS3”.
The CSS is used to better organize a web site. CSS helps the designer to define colors, fonts and sizes, design layout, spaces and other parameters and to edit them from a specific file. this way a change in web presentation can be made by replacing items in the CSS file.
The other good idea besides css is to use include files to import header and footer of your web site. Those can be defined in CSS as well. Using include files eases the designers work by saving time to make changes on, in some cases, thousands of web pages.
Learning CSS is easy. W3 Schools offer CSS tutorials. Other useful web sites that help you with CSS are CSS Play, CSS Beauty, CSSVault and etc.