Ghostscript is the name of a set of software that provides:
An interpreter for the PostScriptTM language, with the ability to convert PostScript language files to many raster formats, view them on displays, and print them on printers that don't have PostScript language capability built in;
An interpreter for PDF files, with the same abilities;
The ability to convert PostScript language files to PDF (with some limitations) and vice versa; and A set of C procedures (the Ghostscript library) that implement the graphics capabilities that appear as primitive operations in the PostScript language.
Ghostscript is a command line oriented program, and hence does not have a very friendly interface. Ghostview is a graphical interface to Ghostscript. You should download and install both programs, and then you actually run only Ghostview. Ghostview runs Ghostscript in the background.