The Joomla Performance Booster component (I don't know if you should call it a component as it runs largely on top of your Joomla! installation) combines, compresses and compacts JavaScripts and CSS files into one file each. This means your CSS files will be put into one file for your browser to load.
There can be a lot of CSS and JavaScripts on your site, depending on the Joomla! template and the number of components you use. Sometimes a component comes with several extra CSS files to handle the layout items for that component. Just for fun you should look at your source code (shown in the following screenshot) to see how many CSS files you are loading at the moment.
The following is a detailed image showing you the javascript and stylesheet calls.
JavaScripts are called using:
<script type="text/javascript" src="/media/system/js/mootools.js"></script>
Stylesheets are called using:
<link rel="stylesheet" href="/templates/template_name/css/template...