When you start Emacs, it normally attempts to load your init file. This is either a file named .emacs or .emacs.el in your home directory, or a file named init.el in a subdirectory named .emacs.d in your home directory.
The command-line switches ‘-q’, ‘-Q’, and ‘-u’
control whether and where to find the init file; ‘-q’ (and the
stronger ‘-Q’) says not to load an init file, while ‘-u
user’ says to load user’s init file instead of yours.
See Entering Emacs in The GNU Emacs Manual. If neither
option is specified, Emacs uses the LOGNAME environment
variable, or the USER (most systems) or USERNAME (MS
systems) variable, to find your home directory and thus your init
file; this way, even if you have su’d, Emacs still loads your own init
file. If those environment variables are absent, though, Emacs uses
your user-id to find your home directory.
Emacs also attempts to load a second init file, called the early init file, if it exists. This is a file named early-init.el in your ~/.emacs.d directory. The difference between the early init file and the regular init file is that the early init file is loaded much earlier during the startup process, so you can use it to customize some things that are initialized before loading the regular init file. For example, you can customize the process of initializing the package system, by setting variables such as package-load-list or package-enable-at-startup. See Package Installation in The GNU Emacs Manual.
An Emacs installation may have a default init file, which is a
Lisp library named default.el. Emacs finds this file through
the standard search path for libraries (see How Programs Do Loading). The Emacs distribution does not come with this file; it is
intended for local customizations. If the default init file exists,
it is loaded whenever you start Emacs. But your own personal init
file, if any, is loaded first; if it sets inhibit-default-init
to a non-nil value, then Emacs does not subsequently load the
default.el file. In batch mode, or if you specify ‘-q’
(or ‘-Q’), Emacs loads neither your personal init file nor
the default init file.
Another file for site-customization is site-start.el. Emacs loads this before the user’s init file. You can inhibit the loading of this file with the option ‘--no-site-file’.
This variable specifies the site-customization file to load before the
user’s init file. Its normal value is "site-start". The only
way you can change it with real effect is to do so before dumping
Emacs.
See Init File Examples in The GNU Emacs Manual, for examples of how to make various commonly desired customizations in your .emacs file.
If this variable is non-nil, it prevents Emacs from loading the
default initialization library file. The default value is nil.
This normal hook is run, once, just before loading all the init files (site-start.el, your init file, and default.el). (The only way to change it with real effect is before dumping Emacs.)
This normal hook is run, once, just after loading all the init files (site-start.el, your init file, and default.el), before loading the terminal-specific library (if started on a text terminal) and processing the command-line action arguments.
This normal hook is run, once, just after handling the command line arguments. In batch mode, Emacs does not run this hook.
This normal hook is very similar to emacs-startup-hook.
The only difference is that it runs slightly later, after setting
of the frame parameters. See window-setup-hook.
This variable holds the absolute file name of the user’s init file. If the actual init file loaded is a compiled file, such as .emacs.elc, the value refers to the corresponding source file.
This variable holds the name of the Emacs default directory.
It defaults to ${XDG_CONFIG_HOME-'~/.config'}/emacs/
if that directory exists and ~/.emacs.d/ and ~/.emacs do
not exist,
otherwise to ~/.emacs.d/ on all platforms but MS-DOS.
Here, ${XDG_CONFIG_HOME-'~/.config'}
stands for the value of the environment variable XDG_CONFIG_HOME
if that variable is set, and for ~/.config otherwise.
See How Emacs Finds Your Init File in The GNU Emacs Manual.