Ghostscript is an incredibly significant program for Linux printing.
Most printing software under Unix generates PostScript, which is
typically a $100-300 option on a printer. Ghostscript,
however, is free, and will generate the language of your printer from
PostScript. When tied in with your lpd input filter, it gives
you a virtual PostScript printer and simplifies life immensely.
Ghostscript is available in two forms. The commercial version of Ghostscript, called Aladdin Ghostscript, may be used freely for personal use but may not be distributed by commercial Linux distributions. It is generally a year or so ahead of the free Ghostscript; at the moment, for example, it supports Adobe Acrobat's Portable Document Format, while the older Ghostscripts do not.
The free version of Ghostscript is GNU Ghostscript, and is simply an aged version of Aladdin ghostscript kindly given to GNU. (Kudos to Aladdin for this arrangement; more software vendors should support free software in this way).
Ghostscript's main failing is in its fonts, which are derived from bitmap scans of the characters. Superior, but commercially licensed fonts may be obtained from several places, since Ghostscript can read type 1 Adobe fonts. Adobe distributes useable fonts with their Acrobat PDF reader, or you may use the fonts from Adobe's ATM or a fontpack.
Whatever you do with gs, be very sure to run it with the option
for disabling file access (-dSAFER). PostScript is a fully
functional language, and a bad PostScript program could give you quite
a headache.