Cascading Style Sheets, or CSS, is a way to describe how to display HTML. CSS has enormous flexability and is not limited to just web browser, which is part of the reason it is so powerful.  You can use CSS to describe how to display content in a web browser, print formation, television display, braille, projection, handheld, or even a speech reader.

Learning CSS can be frustrating at first due to different results in different browsers.  When working with CSS, you will constantly run into cross-browser compatibility issues.  However, if you stick with W3C standards and always include a DocType declaration on all of you page, you will find that you can over come this issues and fully exploit the power of CSS.