🚀 go-pugleaf

These concepts surface in the GNU newsgroups from time to time. I have
a vague idea (which may have no relation to reality whatsoever ;-)
about what they are:
  1. emacsclient: This is a "lightweight" emacs that have only a few
                  buffers of its own (only one perhaps?). It edits a
                  text file/text buffer piped down to it through a
                  socket connection/pipe from another program. It has
                  no file operations of its own(?).
  2. emacsserver: This is a server process that maintains buffers,
                  reads and writes files, runs emacs lisp commands,
                  controls sub-processes and creates/destroys
                  emacsclients at need.
  3. gnuserv    : The original emacs server, written by Andy Norman.

Can somebody clarify these concepts for me? I am especially interested
in emacsclient, since I saw that it could be used as a support editor
for other programs (xrn mentions it as an example for an editor to use
for composing messages). How lightweight is it? How *emacs* is it?
Can it be modified using emacs lisp (modes, keymaps, etc.)? If so:
Where does it get the lisp code? From the parent process? From the
~/.emacs file?

Thanx in advance for any and all answers!

- Steinar