What Is Windows Sidebar?

Back when Microsoft shipped Windows 98, it added a debatably useful feature called Active Desktop that provided an HTML (Hypertext Markup Language) layer on top of the traditional desktop. Active Desktop was an attempt to capitalize on the then-emerging trend of users wanting to combine live data from the Web with their PC operating system.
The term for this at the time was push technology. The idea was that although you could use a Web browser to manually find data on the Web, or pull data from the Web, a pushtechnology client like Active Desktop could push data to the user automatically with no interaction required.

Ultimately, users found Active Desktop to be confusing and undesirable, mostly because
Microsoft and its partners used it as a front end for advertisements and other unwanted information; and although the feature was never really removed from Windows, it was deemphasized in subsequent Windows versions, such as Windows XP. However, it’s still possible to add Web content to your XP desktop via Active
Desktop if you really want to.

Active Desktop may have failed, but the underlying benefits of push technology are still valid today. You can see that this type of functionality still exists in such technologies as RSS (Really Simple Syndication), which in fact attempts to solve essentially the same problem as Active Desktop: Rather than force users to manually search for the content they want, that content is delivered automatically to them using a unique kind of client
(in this case, an RSS client).

In Windows Vista, Active Desktop is finally gone forever, but integrated push technology lives on with a brand-new feature called Windows Sidebar. Like Active Desktop,
Windows Sidebar is available by default and is running when you start up your new Windows Vista–based PC for the first time. Moreover, it will keep on running unless you configure it not to do so, but Windows Sidebar solves one of Active Desktop’s major problems by moving the main user interface off the desktop and to the side of the screen where it won’t typically be hidden under your open applications and other windows. (If it is hidden by windows, you can optionally configure Windows Sidebar to appear “on top” of other windows.)

Source of Information : Wiley Windows Vista Secrets SP1 Edition