Part of the new Windows Phone and Windows Mobile 6.5 operating system is a feature called widgets.
Bill Fisher from http://www.windowsphoneconnection.com describes the widgets as small applications that are meant to accomplish just one simple thing, such as pulling weather or financial information from the Web. Thanks to the new and improved rendering engine of the new Internet Explorer Mobile, Windows phone supports widgets.
Widgets appear like any normal application in your Start screen, but they are actually reliant on the Web for their data. These little apps are very easy to develop, requiring only knowledge of AJAX, HTML, CSS, or other Web-development languages. They will also be available in the Windows Marketplace for Mobile.
SOURCE: Windows Phone Connection