The macro language of TeX refers to the way macros are read, defined, expanded, etc.; the set of primitive commands & their effect; and things like that. In this context, “plain TeX” means the stuff DEK’s engine understands, as opposed to the extensions supplied by engines like e-TeX, XeTeX, PDFTex, LuaTeX, etc., etc.
The format is the way TeX is configured: which characters have which catcodes, what names do the primitives have, and which macros are pre-defined and to what. In this context, “plain TeX” is the format described in The TeXBook, as opposed to extensions like
eplain, LaTeX, & ConTeXt.
The difference between a format and a macro package is not quite clear-cut: there are commands to turn the currently-loaded set of macros into a new format. This is how LaTeX and ConTeXt work without requiring something like
\input latex at the top of your source.
By convention, the
tex command loads the TeX engine with the “plain” format.