What is a common printer interface today? I've read most of them support PostScript and/or PCL. How should I use PS/PCL from an app code (say, under Win32)?
PostScript and PCL are both open-spec as far as I am aware. The definitive source for PostScript would be the PLRM (PostScript Language Reference Manual). PCL's equivalent can be found at this page.
Most modern multi-function printers for office environments will accept a number of different PDLs. PostScript and PCLXL are the most common, but some others are:
Many manufacturers also implement a proprietary PDL. Since PostScript, PDF and XPS can be slow to parse and render, a manufacturer often implements a proprietary PDL that is optimised for the printer's hardware and firmware. A lot of manufacturer-rated page-per-minute counts are only possible if you use their custom PDL driver. This technique is also used by low-end budget printers where the hardware is incapable of interpreting high-level PDLs.
In terms of generating this output, usually you do not need to do so, instead you should go through Windows GDI, or depending on your target OS, GDI+. Your drawing and text-output routines will be handled by a printer driver which in turn will generate output for a specific printer. There are also generic drivers out there (Microsoft includes a generic PostScript printer driver) that can be used to generate output that is not specific to any particular printer.