TeX is open source and thus at home on “all” systems.
TeX was published in 1982; version number is approxing Pi. The very last error was found in 2002 by NTS project.
TeX the program is the typographical engine used by several automatical systems for their high-quality output. Originally it was written in WEB and Pascal. Nowadays mostly a C-based version is used. NTS was a new implementation in Java. The most important modern version is LuaTeX.
TeX the language is an extensible macro language, specialised on typesetting. Prof. Knuth himself wrote the PlainTeX extension, basis for mighty and complex systems like LaTeX and ConTeXt.
TeX the lion is our lovable mascot, drawn by Duane Bibby. His mate is Meta the lioness, like METAFONT (and its derivate METAPOST) are TeX’s companions).