PicTeX is a set of macros by Michael Wichura for drawing diagrams and pictures. The macros are freely available; however, the PicTeX manual itself is not free. Unfortunately, TUG is no longer able to supply copies of the manual (as it once did), and it is now available only through Personal TeX Inc, the vendors of PCTeX (http://www.pctex.com/). The manual is not available electronically.
However, there is a summary of PicTeX commands available on CTAN, which is a great aide-memoire for those who basically know the package to start with.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=docpictex