Most people call them widgets, but Google wanted to be different: the first name was modules, but it was changed to gadgets. They're wrappers for small pieces of content from the web that can be added to a web page or to the desktop. "Google Gadgets are mini-applications that work with iGoogle, Google Desktop, or any page on the web. They can range from simple HTML to complex applications, and can be a calendar, a weather globe, a media player, or anything else you can dream up."
Since then, Google's gadgets evolved a lot: they have more functionality, can be added to web pages and Google Desktop, became a new form of advertising and are able to recreate a web page from scratches. But gadget also became synonymous to extension, add-on, plug-in at Google. The concept was extended to other applications: Google Maps (mapplets), Google Calendar (web content), Google Toolbar (buttons), Google Search (subscribed links) and more.
Google uses similar directories for all these different kinds of gadgets:
* iGoogle gadgets (more than 20,000)
* Google gadgets for web pages (a subset of the iGoogle gadgets)
* Google Desktop gadgets
* Google Toolbar buttons
* Google Maps mapplets
... but also for other types of content:
* Google Desktop plug-ins (extend the indexing capabilities)
* Google Earth KML files (overlays)
While most iGoogle gadgets can be added to web pages, Google Desktop gadgets can be added to iGoogle only using a browser plug-in and mapplets work only inside Google Maps. There's still no single standard for gadgets and you need to write different code, depending on the medium where the gadgets run.
It would be nice to write a single code that displays status data in Google Toolbar or in a mobile app, a simple interface in iGoogle, a richer interface in Google Desktop's sidebar and could also integrate with Google Maps or Google Calendar. A mail gadget could show the number of unread messages in Google Toolbar, their subjects in iGoogle, their content in Google Desktop, the locations mentioned in the messages at Google Maps and the events at Google Calendar. You would only need to add the gadget once and see it in different incarnations, depending on the context.