The acronym CSS stands for Cascading Style Sheets. CSS is used to define how to display XHTML elements.
Historically HTML was originally designed to define the content of a document. The display of the document was indented to be handled by browsers without using formatting tags. However, browser manufacturers added new HTML tags and attributes to the HTML specification. As a result it became more difficult to create Web Sites where the content was separated from the presentation.
CSS is a method of separating presentation from content. CSS styles define how XHMTL elements are displayed or look. CSS allows for the style and layout control of an entire site to be isolated in one location simplifying global changes.