Desktop publishing, or DTP, is the process of editing and layout of printed material intended for publication, such as books, magazines, brochures, and the like using a personal computer. Desktop publishing software, such as QuarkXPress, is software specifically designed for such tasks. Such programs do not generally replace word processors and graphics applications, but are used to aggregate content created in these programs: text, bitmap graphics (such as images edited with Adobe Photoshop, and vector graphics (such as drawings/illustrations made with Adobe Illustrator).
Desktop publishing was invented in 1985, with the conjunction of Aldus Pagemaker (later acquired by Adobe), the Apple Macintosh, and Adobe Systems' PostScript page description language, including its scalable fonts in Type 1 format.
Until 1978 typesetting was performed by mechanical or electro-mechanical means, or by extremely expensive mainframe or mini-computer based systems. In that year the TeX program showed that publication-quality typesetting could be done on any normal business computer, and even long and complex jobs like books and journals could be produced from a standard desktop terminal.
When personal computers acquired graphical screens, synchronous typographical editing became possible (incorrectly known as "WYSIWYG"), but these systems were initially used mainly for small-distribution publications such as club newsletters. As these systems improved they became widely adopted throughout the professional publishing world -- presently, virtually all publishing is "desktop publishing". The superior flexibility and speed of desktop publishing systems has greatly reduced the lead time for magazine publication and allowed more elaborate layouts than would otherwise have been possible. Programmable, automated systems like LaTeX mean that long, repetitive, or highly-structured documents can be produced in a fraction of the time that it would take a manually-controlled system.
The Apple Macintosh, with historically superior graphics capabilities (particularly in the area of typography), and a simple GUI, is highly popular in this application domain and remains one of Apple's core markets.