Uranium is a group of widgets - but what is a widget, exactly? In Uranium, a widget is a behavior that you're assigning to HTML elements on a page. It's a group of attributes you give to your HTML, allowing it to perform a function.
Each widget page will give a brief description of the widget followed by the attributes necessary to add to your HTML (and where to put them). Finally there will be a demonstration of the widget with its HTML to illustrate the layout of the widget.
If you're a bit lost with these pages, you might want to check out the tutorial page, which takes you step-by-step through an example of implementing a widget.
Not all widgets work perfectly with every browser or device. Check out the compatibility charts.