From other answers on this site and the FAQ of the TeX User Group my understanding is that:
- TeX is the original typesetting program, written by Knuth
- Omega, eTeX, pdfTeX, XeTeX, LuaTeX are engines. They are variations of the original TeX program, written in different languages, designed to give almost the same output for the same input as the original TeX, but with great variations in the inner workings and slight variations in the interface that make possible for the user to access the inner workings
- LaTeX and ConTeXt are macro packages. They are like libraries of TeX macros. While LaTeX works on all the engines, the last version of ConTeXt (Mark IV) works only with LuaTex, because it uses many features exclusive of this engine.
What I do not understand is why both LaTeX and ConTeXt require their own executable. Isn't it possible just to include in the TeX source files where one wishes to use LaTeX just the definitions of the LaTeX macros (maybe with an