The program is primarily used to design short documents (i.e. not books), posters and other print media, but it also has some multimedia/web/pdf features. Longer documents like books are usually still designed with FrameMaker.
InDesign is slowly replacing the deeply entrenched Quark XPress as the page layout program preferred by professionals. Most university design programs now teach InDesign instead of Quark, but many professional designers have not switched. This article from MacWorld compares the two programs. Note that since that article was published the new version 3.0 of InDesign (dubbed InDesign CS) was released with many significant improvements.
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