TeX is a batch formatter similar to troff developed by Don Knuth
at Stanford.  It is most popular in universities and is freely
traded because of the lack of licensing restrictions. ArborText
sells a commercial version for big and small (ie PCs) machines.
There are various versions for PC-class machines. LaTeX (Leslie
Lamports version of TeX) is a more friendly version of the same
formatter. It does for TeX what macros does for troff.  See
comp.text.tex for TeX FAQs.

[To be pedantically correct, there are really only very few versions
of TeX.  However, the relationship between the formatter and
the macro sets (e.g.: LaTeX) is considerably more blurry in TeX than
troff, and the macro package tends to be compiled-into different
instantiations of TeX.  This is truly part of the culture - TeXies
refer to "latex documents", troffers more generally refer to
"troff with mm macros".  It's confusing.]

Oh, and to save yourself sounding like an idiot, the "eX" in "TeX"
is pronounced so that it rhymes with "blecchh", not "sex" (the
X is a greek chi).