TeX
TeX (the X stands for the greek Χ letter and so pronounced as ch in technics, and written with a lowercase 'e' in imitation of the logo) is a typesetting language and system designed and mostly written by Donald E. Knuth.
See wikipedia TeX for more.
Normally a bare TeX is hardly usable to typeset, so macro packages are often used. The basic macro package given as default is Plain TeX; a very common and widely used macro package is LaTeX, but there are many.