A PS file is an image saved in the PostScript page description language. It may contain vector graphics, raster graphics, and text. The file can be printed directly by a PostScript printer (i.e., containing a Raster Image Processor) without being opened in an application.
PS files have largely been replaced by other page layout file formats, such as .PDF. Yet, they are still supported by many printers and programs. In general, they can be opened by any printer utility that supports PostScript.
NOTE: You can convert PS files to PDFs using Acrobat Distiller, a program included with Adobe Acrobat.