Emacs uses keymaps to record which keys call which commands.
When you use global-set-key to set the key binding for a single
command in all parts of Emacs, you are specifying the key binding in
current-global-map.
Specific modes, such as C mode or Text mode, have their own keymaps; the mode-specific keymaps override the global map that is shared by all buffers.
The global-set-key function binds, or rebinds, the global
keymap. For example, the following binds the key C-x C-b to the
function buffer-menu:
(global-set-key "\C-x\C-b" 'buffer-menu)
Mode-specific keymaps are bound using the define-key function,
which takes a specific keymap as an argument, as well as the key and
the command. For example, my .emacs file contains the
following expression to bind the texinfo-insert-@group command
to C-c C-c g:
(define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group)
The texinfo-insert-@group function itself is a little extension
to Texinfo mode that inserts ‘@group’ into a Texinfo file. I
use this command all the time and prefer to type the three strokes
C-c C-c g rather than the six strokes @ g r o u p.
(‘@group’ and its matching ‘@end group’ are commands that
keep all enclosed text together on one page; many multi-line examples
in this book are surrounded by ‘@group … @end group’.)
Here is the texinfo-insert-@group function definition:
(defun texinfo-insert-@group () "Insert the string @group in a Texinfo buffer." (interactive) (beginning-of-line) (insert "@group\n"))
(Of course, I could have used Abbrev mode to save typing, rather than write a function to insert a word; but I prefer key strokes consistent with other Texinfo mode key bindings.)
You will see numerous define-key expressions in
loaddefs.el as well as in the various mode libraries, such as
cc-mode.el and lisp-mode.el.
See Customizing Key Bindings in The GNU Emacs Manual, and Keymaps in The GNU Emacs Lisp Reference Manual, for more information about keymaps.