TeX is a typesetting system invented by Donald Knuth
with the help of many others (including, but not restricted to, Scott Kim, Hermann Zapf, Matthew Carter, Kris Holmes,
Charles Bigelow, etc.) It is used in a wide variety of projects, though to the new user it may seem dauntingly
perverse, and unnecessarily complex.
Knuth has frozen the development of TeX, and every three years he reviews bugs. TeX is presently at version 3.14159,
and metafont at 2.718. He will issue a finder's fee for every bug found in these computer programs. Upon Knuth's untimely death (whenever that will occur), TeX will be at version $\pi$, and metafont will be at version $e$, representing (respectively), the ratio of the circle's circumference to its diameter, and the base of the natural logarithms
As the development of TeX and metafont has stopped, a number of extensions to TeX have been proposed. eplain, etex, and elatex are modifications to the core of TeX which
are written in the same language that TeX is written in, and (in most cases) process .tex source files identically to tex or latex respectively. Omega is another extension to the core of TeX, implementing unicode input and output
The language that TeX source files are written in
should be distinguished from what the tex program
(and other associated programs) do. Each entity on
the page is a box and TeX arranges these boxes (with
the use of a pretty clever line breaking algorithm)
into pleasing patterns, ensuring that the text is neither
too bunched up or too spread apart. Metafont's role
is to draw the shapes of each character
There are a series of macro extensions to TeX,
including (but not limited to -- see CTAN) LaTeX,
Context, Lollipop, amongst others
you can use postscript fonts with TeX. the learning
curve is awfully steep though