CSS is the key presentational technology that is used in website design.
CSS stands for Cascading Style Sheets. CSS is a standard style sheet language used for describing
the presentation (i.e. the layout and formatting) of the web pages.
Prior to CSS, nearly all of the presentational attributes of HTML documents were contained within the
HTML markup (specifically inside the HTML tags); all the font colors, background styles, element alignments,
borders and sizes had to be explicitly described within the HTML.
As a result, development of the large websites became a long and expensive process, since the style
information were repeatedly added to every single page of the website.
To solve this problem CSS was introduced in 1996 by the World Wide Web Consortium (W3C), which
also maintains its standard. CSS was designed to enable the separation of presentation and content.
Now web designers can move the formatting information of the web pages to a separate style sheet which
results in considerably simpler HTML markup, and better maintainability.
CSS3 is the latest version of the CSS specification. CSS3 adds several new styling features and
improvements to enhance the web presentation capabilities.
There are lot more things you can do with CSS.