Plain TeX is the common base platform for all of the TeX typesetting programs. A limited range of support files are available for Plain TeX, which includes the pstricks package.
The input files that are available for TeX as support include
Options for use with dvips
Extended plain TeX options
TeX draw input files
Local TeX options
In the absence of any sensible documentation mechanism for plain TeX, the links above are to the TeX source files themselves, so you can work out what they do.
