Unfortunately this extension can mean one of two slightly incompatible things: an Apple Dashboard Widget or an Opera Widget. In each case it'll function a lot like an applet desktop accessory. In the former case it will work on computers running Mac OS X or KDE. In the latter case it will work on computers running the Opera browser. The Apple Widget will consist of a group of files kept together in an application bundle; the Opera Widget will consist of a group of files zipped together. See also the similar widget extension.