What is Eplain?
The Eplain macro package expands on and extends the definitions in Plain TeX. Eplain is not intended to provide “generic typesetting capabilities”, as do ConTeXt, LaTeX or Texinfo. Instead, it defines macro tools that should be useful whatever commands you choose to use when you prepare your manuscript.
For example, Eplain does not have a command
which would format section headings in an “appropriate” way, as
\section does. The philosophy of Eplain is that
some people will always need or want to go beyond the macro designer's
idea of “appropriate”. Canned sets of macros are fine — as long as you
are willing to accept the resulting output. If you don't like the
results, or if you are trying to match a different format, you may
find that Eplain is for you.
On the other hand, almost everyone would like capabilities such as cross-referencing by labels, so that you don't have to put actual page numbers in the manuscript. The authors of Eplain believe it is the only generally available macro package that does not force its typographic style on an author, and yet provides these capabilities.
Another useful feature of Eplain is the ability to create PDF files with hyperlinks. The cross-referencing macros can implicitly generate hyperlinks for the cross-references, but you can also create explicit hyperlinks, both internal (pointing to a destination within the current document) and external (pointing to another local document or a URL).
Several LaTeX packages provide capabilities which Plain TeX users are lacking, most notably text coloring and rotation provided by the graphics bundle (packages color and graphics). Although the graphics bundle provides a Plain TeX “loader” for some of the packages, it is not a trivial job to pass options to those packages under Plain TeX, and much of the functionality of the packages is accessed through package options. Eplain extends the loader so that options can be passed to the packages just as they are in LaTeX. The following packages are known to work with Eplain: graphics, graphicx, color, autopict (LaTeX picture environment), psfrag, and url.
Source: What is Eplain?