Ghostscript is an interpreter for the PostScript (TM) language. A PostScript interpreter usually takes as input a set of graphics commands. The output is usually a page bitmap which is then sent to an output device such as a printer or display. PostScript is embedded in many printers.

Ghostscript has several main uses:
Display a PostScript file (avoid killing trees).
Display a PostScript file to decide if you really need to print it (reduce the number of trees killed).
Print a PostScript file to a non-PostScript printer (kill more trees).