Ghostview would be a Linux peice of software, probably aviable for Windows also.
Ghostscript is a whole open source postscript and pdf implimentation. Tools, 'distillers' and all sorts of stuff. Do things like convert PDF files to text, extract images, create new pdfs, etc etc.
For instance at work we'll use it to convert data sent to a special email address in text form which is automaticly into a pdf which then is made aviable to costumers through the ftp site. Good for automation and stuff like that.
Another common use would be for technical documentation. People write stuff in TeX document format (a markup language sort of like html, but made for word proccessing) then they will convert it into a veriaty of formats. A very common format would be PDF. Good for stuff that requires a lot of mathmatical symbols and such.
http://www.latex-project.org/
http://www.lyx.org/
http://www.cs.wisc.edu/~ghost/
Good for ultra-high quality professional documentation. Not realy usefull for normal stuff like a regular word proccessor. Probably something I'd use to make resumes though.
Also besides stuff like that you have it in the form of PDF output for OpenOffice.org.
It's tied pretty closely to postscript, so people have used ghostscript stuff to make things like PDF printers.. So you can send printouts to a print server on your network over Samba or IPP or something like that, but instead of printing out paper it will make PDF files.
people have written fancier programs with it besides that. For instance there is a program called Hylafax were you can setup a computer to send and receive faxes over a telephone line, but instead of putting in paper and such you email and recieve faxes in the forms of PDFs or Tiff files or something.
For Linux there are a veriaty of other PDF viewers besides Ghostview.
Ghostview is fast, but it's pretty ugly and not very user friendly.
Personally I prefer Evince,
http://www.gnome.org/projects/evince/
I like it because it's fast and has smooth scaling so that the pages adjust well to different sizes and such.
I don't think they have a version for Windows yet though. They might.
