The groff package provides compatible versions of troff, nroff, eqn, tbl, and other Unix text-formatting utilities.
Groff is used to "compile" man-pages stored in groff/nroff format for different output devices; e.g., for displaying to a screen or in PostScript(tm) format for printing on a PostScript(tm) printer. Most programs store their man-pages in either /usr/man/ or /usr/X11R6/man/.