TeX is a command-based typesetting system by Prof. Donald E. Knuth.
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).
ConTeXt is a modern, mighty and easy variant of TeX. It was developed mainly by Hans Hagen (Pragma ADE) and Taco Hoekwater from the Netherlands.
ConTeXt is the layout-oriented alernative to the well-known LaTeX.
ConTeXt is popular as a XML processor. Usabel are e.g. XHTML, MathML, PhysML and ChemML. Since 2010 also the direction from TeX to XML is possible (e.g. for Tagged PDF or PDF/A).
ConTeXt handles prepress requirements like imposition, spot colors and grid setting.
ConTeXt normally uses PDF as output format and can employ the possibilities of current PDF versions (widgets, transparency, layers, JavaScript, multimedia).
There’s a more extensive in presentation in Wikipedia.