Aladdin GhostScript is a software-based, device independent implementation of Adobe's PostScript and Portable Document Format page description languages.
It converts PS or PDF files to device formats, and can do conversions between PS and PDF. However, the GPL version doesn't seem to do as good job as Adobe's Acrobat: It renders everything else except the core Adobe fonts (Times, Helvetica, Courier and the like) as graphics, resulting terabyteful PDF files. (Then again, it's often a good idea to use those fonts anyway, but...)
The "gs" executable that is the heart of GhostScript is just the heart of the thing that does the PS/PDF-to-device-format conversion. The output in device format can be sent to printer (this is the way GS is used most), or other programs can be used to display the rendering on screen (ghostview, gv, and the equivalents in GNOME and KDE, whatever they're called).