This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
These programs offer features lacking in word processors, usually tailored to flexible arrangement of text, but often don't provide all of the text manipulation that a good word processor provides. The best desktop publishing programs strive to provide the features of both, just as the best word processors strive to provide the features of desktop publishing programs. Microsoft Word (Mac, PC) is a good example of a word processing program that offers many page layout feature. Framemaker (UNIX, Macintosh, DOS, etc.) is an example of a desktop publishing system that offers most needed word processing functions. As yet, no Amiga program has bridged the gap, though the main word processors are coming close. (On the other hand, even many sophisticated programs don't support typesetting mathematics, tables, producing bibliographies, indexes, or cross-references. The page layout languages do, and programs like Frame are improving their support of such features.) Unless you need to prepare fancy newsletters or promotional literature, a word processing program is probably enough. See Word Processors.
There are not yet any freely distributable wysiwyg desktop publishing systems. Commercial products are ProPage and PageStream. They have been playing leapfrog for the past few years. It appears that PageStream 3.0 is about to leap ahead. A more detailed description of these products and their differences is welcome. Both programs' list prices are $299. Student discounts are available (approx 40% discount.)