This is an example of a specially-tagged web page that contains an HTML widget and uses AddThis for sharing. An HTML widget is just a small web page that's designed to be viewed in an iframe that is embedded on another web page such as a start page or desktop. View the source to see how it all works, or read an overview of sharing widgets with AddThis.
This page contains special tags that identify the HTML widget's properties so AddThis and others can locate it (view the page source to see them). These tags specify the iframe URL, height, width, and other content properties.
This is a widget in an HTML iframe:
When AddThis shares the page to a destination that supports HTML widgets, it will locate the widget and share it along. Share it to iGoogle to see this in action: