A coding language that allows the person creating the site to separate the design elements from the content of the site. Before CSS if someone wanted, say the text to be displayed in a certain font at a particular size they would have used something called a font tag. This would be around every paragraph of text that you wanted to be in that particular style...
<font size="3" face="Verdana"> <p>Here is some text at font size 3 displayed using the Verdana font.</p> </font>
Now, supposing you wanted to change the font used throughout your site to Times? This would require changing every font tag reference of Verdana to Times. With CSS these styles are in a separate file called a Style Sheet which is loaded in at the beginning of every HTML page. In this page you can set and therefore change the style of the text used throughout the site from one place which also makes the HTML code much neater...
<p>Here is the same text but this time it's style is defined in a separate global file.</p>
CSS is much more powerful than this as it also allow the designer full control over positioning of any element on the site. Images can be displayed via the Style Sheet as a background image which are loaded in and saved in a special area of memory used by the Web Browser called a Cache which means they don't need to be downloaded each time they are used on a new page, thus making the site load quicker.
Find out more about CSS, and see just some of the cool things you can do with CSS.