A widget– or "gadget," as they are called in the new Microsoft Vista operating system – is a small, specialized application you can download and run on your desktop. Some of the most common widgets and gadgets are clocks, battery monitors, calculators, calendars, news aggregators, and weather forecasts. More specialized widgets and gadgets even let you track flights or monitor traffic on your route home.

Because they don't take up much space, look fun, and handle live data well, widgets and gadgets can be an alternative to multiple open browsers or system tools.

Gadgets will be supported natively within Microsoft Vista. Windows XP (Service Pack 1) and Windows 2000 (Service Pack 3) run widgets using the Yahoo! Widget Engine. Apple computers run Yahoo! Widgets as well as their own native widgets on Mac OS X v10.4.

More Information on Widgets:

Wikipedia

Yahoo Widget FAQ

Apple Dashboard

Microsoft Gadgets