General/TeX is a typesetting system written by Donald E. Knuth [http://sunburn.stanford.edu/~knuth/]. It is often used for the processing of mathematical and scientific texts.
Wikipedia has a good page on General/TeX: http://www.wikipedia.org/wiki/General/TeX
General/TeX for the Impatient, The book by Paul W. Abrahams, Kathryn A. Hargreaves, and Karl Berry is now available (September 9, 2003) in open source form. The pdf-file is available: http://www.tug.org/ftp/tex/impatient/book.pdf
There is a very good front end for General/TeX and General/LaTeX for OS X available from: http://darkwing.uoregon.edu/~koch/texshop/texshop.html