LaTeX and ConTeXt are two packages that provide sets of macros for the base TeX typesetting system. LaTeX provides macros for things like sectioning, tables of contents, figures, and so on, and the configurable styling of everything separately from the content; ConTeXt's macros are more focused on precise page layout than semantic document structure.
TeX Live is a TeX distribution, meaning it contains TeX, LaTeX, ConTeXt, and a variety of other macro packages (including packages building on top of LaTeX, for example), fonts, and so on.
TeX live is a distribution and it comes with many things (TeX, LaTeX, ConTeXt, fonts, ...). Both LaTeX and ConTeXt are built on top of bare TeX and add lots of features. I have used ConTeXt a quite a bit, and it seems to be more designed to typeset whole books, whereas LaTeX is very good at scientific papers.
