In the FSS, a "widget" is a small collection of elements grouped together in a particular, common layout. A widget typically includes a title bar, a few controls, and a content area. Typical uses of a widget might be to group together a set of website tools or favourite links in a sidebar: the content area would list the tools or links, the controls would allow users to add or perhaps sort the list, etc.
The FSS widget classes can be used to help style such a widget. They will apply layout styles to particular elements:
<h2> inside the title bar
<ul> inside the options
On their own, they don't do much more than this, but in conjunction with the FSS themes, they begin to shape the appearance of the widget.