PostScript is a general-purpose programming language, like C or Pascal. In addition to providing the usual features of a language, however, PostScript allows a programmer to specify the appearance of both text and graphics on a page.
A PostScript printer is equipped with a computer that runs an interpreter for processing PostScript language files. When a PostScript printer receives a file, it runs that file through the interpreter and then prints it. Unless special provisions have been made by the manufacturer, files submitted to a PostScript printer must be written in the PostScript language.
Why would you want to use a PostScript printer? PostScript provides excellent facilities for managing text and graphics and combining them. Also, most major applications that support printed output support PostScript. Graphics operators facilitate the construction of geometric figures which can then be positioned and scaled with any orientation. The text capabilities allow the user to specify a number of different fonts that can be placed on a page in any position, size, or orientation. Because text is treated as graphics, text and graphics are readily combined. Moreover, the language is resolution and device-independent, so that draft copies can be proofed on a low-resolution device and the final version printed in higher resolution on a different device.
Applications that support PostScript, including word-processing and publishing software, will create documents in the PostScript language without intervention by the user. Thus, it is not necessary to know the details of the language to take advantage of its features. However, standard files that some applications or special terminals produce cannot be printed on a PostScript printer because they are not described in the language. The print service provides optional filters to convert many of these files to PostScript so that users may take advantage of PostScript and continue to use their standard applications, such as troff.
Retail Type 1 fonts can be installed for use with applications running on the desktop. These fonts may be downloaded to PostScript printers if the application generates PostScript output that uses them. The lp command handles this automatically using the download filter.