Ghostscript is the name of a set of software that provides:

• An interpreter for the PostScript (TM) language and the Adobe Portable Document Format (PDF -- sometimes confused with Acrobat, Adobe's PDF browser/editor product), and
• A set of C procedures (the Ghostscript library) that implement the graphics and filtering (data compression / decompression / conversion) capabilities that appear as primitive operations in the PostScript language and in PDF.

http://pages.cs.wisc.edu/~ghost/doc/GPL/index.htm