This manual page is not meant to be exhaustive. The complete documentation for this version of TEX can be found in the info file or manual Web2C: A TeX implementation.
eplain is an extension of plain TEX described in the book TEX for the Impatient. It adds these features (among others) to plain TEX:
tables of contents
hrule and vrule with a different default than 0.4pt
time of day
verbatim file listing
blank and black boxes
TEXable documentation of etex macros in texinfo format.
Online GNU Emacs info(1) files produced from eplain.texi.