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Printing languages, also sometimes called page description languages, are representations of exactly what needs to be on the screen or printed page. They are generally a collection of drawing commands that programs can generate, often with extra features to make drawing complex pictures or doing fancy things with text easier.
PostScript was developed by Adobe in the early 1980s to provide some sort of generic page description language. It is a fairly complete language; I've written complex PostScript programs by hand. This makes it much easier to write software that can generate PostScript output. Modern troffs can generate PostScript, and ghostscript can be used to process PostScript into printer-specific output for certain non-PostScript printers, so PostScript is a very useful output form.
Printer Command Language, or PCL, was originally developed by Hewlett-Packard, also in the early 1980s, to provide a generic printer language for their entire range of printers. Early versions were very simple, but PCL 3 was sophisticated enough that other printer manufacturers started to emulate it, and it became a de facto standard. PCL's more current incarnations are quite flexible and capable. Incidentally, ghostscript can turn PostScript into PCL, and most printers that can't speak PostScript can speak some form of PCL these days. My primary printer these days speaks PCL 5E, and I use it from both Windows machines and Unix machines.
DVI stands for "device independent" and is the primary output from TEX (and thus LATEX). Like PostScript, it's a generic language for describing the printed page. There are converters that convert DVI into PostScript, PCL and PDF.
PDF is Adobe's successor to PostScript. PDF has a special place on the web, because it's been promoted as a way to distribute documents on the web and have them displayed consistently in a wide variety of environments, something not possible in HTML. This consistency is possible for the same reasons any page description language can provide it: the focus of such a language is on describing exactly what the page should look like rather than being human readable or editable, like most markup languages. However, Adobe has provided Acrobat Reader free for multiple platforms and promoted PDF extensively, so it is the de facto standard for page description languages on the web these days.
-- DJPH
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| 45.13. Formatting Markup Languages -- troff, LATEX, HTML, and So On |  | 45.15. Converting Text Files into a Printing Language |
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