setting the frame title

The 'frame title' (window title) that emacs uses in graphical environments
defaults to something like emacs@hostname.

Of course emacs lets us customize this, by changing the value of
frame-title-format. Emacs accepts many different things there, (see the
documentation for frame-title-format and mode-line-format for that), but
let's look at an example.

Instead of the default emacs@hostname, I find it more useful to include the
name of the file I'm working on instead, or, in case of non-file buffers, the
buffer name. To do this, I have something like the following in my .emacs:

(setq frame-title-format
  '("" invocation-name ": "(:eval (if (buffer-file-name)
                (abbreviate-file-name (buffer-file-name))
                  "%b"))))

As you see, frame-title-format is a template for the items that are present
in the title bar; i.e.. emacs concatenates the items in the list, and it
supports various %-constructs, which are replaced with actual values; see
below.

In addition to the %-constructs, you can use :eval to make emacs evaluate
the expression whenever it wants to update the title bar.

invocation-name is the name of the emacs binary.

abbreviate-file-name replaces the home directory part in file names with
~; for very deep paths it might be nice to do some abbreviation as well as
some shells do; this is left as an exercise to the reader :)

You can experiment with some other things to put in frame-title-format;
use the :eval construct as above to use emacs-lisp functions, and the
various %-specifiers which are replaced by certain values; the emacs
documentation lists the following:

%b -- print buffer name.      %f -- print visited file name.
%F -- print frame name.
%* -- print %, * or hyphen.   %+ -- print *, % or hyphen.
      %& is like %*, but ignore read-only-ness.
      % means buffer is read-only and * means it is modified.
      For a modified read-only buffer, %* gives % and %+ gives *.
%s -- print process status.
%i -- print the size of the buffer.
%I -- like %i, but use k, M, G, etc., to abbreviate.
%p -- print percent of buffer above top of window, or Top, Bot or All.
%P -- print percent of buffer above bottom of window, perhaps plus Top,
      or print Bottom or All.
%n -- print Narrow if appropriate.
%t -- visited file is text or binary (if OS supports this distinction).
%z -- print mnemonics of keyboard, terminal, and buffer coding systems.
%Z -- like %z, but including the end-of-line format.
%e -- print error message about full memory.
%@ -- print @ or hyphen.  @ means that default-directory is on a
      remote machine.
%[ -- print one [ for each recursive editing level.  %] similar.
%% -- print %.   %- -- print infinitely many dashes.
Decimal digits after the % specify field width to which to pad.

So, if we'd like to include the host (system) name and some indication of the
status of this buffer, we could do something like:

(setq frame-title-format
  '("emacs%@" (:eval (system-name)) ": " (:eval (if (buffer-file-name)
                (abbreviate-file-name (buffer-file-name))
                  "%b")) " [%*]"))

Of course, some of the information is available elsewhere already, but it
might be clearer in the frame-title. Or not – there's a lot of room for
tweaking and experimentation here.