A widget is a piece of reusable code that you can plug into virtually any website. A gadget acts just like a widget, often fulfilling the same purpose, but it is proprietary. It only works on a certain website or a specific set of websites.
For example, Google Gadgets can look and act like widgets. But they only work on Google pages.
A widget, on the other hand, works on any page that lets you add an HTML block. You can put them on your blog, or your personalized start page, or your personal website.
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Turn any web widget into a Vista Gadget