Widgets are mini-applications that are usually designed to perform a single function. Mac OS X comes with some single-function widgets, including Dictionary, Address Book, Calculator, Calendar, and Flight Tracker.
Mac users who have been around long enough to remember Mac OS 9 and older versions of the Mac OS may find that widgets remind them of desk accessories. While similar in function to desk accessories, widgets are different in an important way. Widgets run in a special layer of the operating system that prevents them from accessing system-level events. This makes widgets stable, as well as a low security risk.
You can download widgets from Apple's web site, as well as create your own widgets, using basic web development methods.