When Emacs is started, it normally loads a Lisp program from the file `.emacs' in your home directory. We call this file your init file because it specifies how to initialize Emacs for you. You can use the command line switches `-q' and `-u' to tell Emacs whether to load an init file, and which one (see section Entering and Exiting Emacs).
There can also be a default init file, which is the library
named `default.el', found via the standard search path for
libraries. The Emacs distribution contains no such library; your site
may create one for local customizations. If this library exists, it is
loaded whenever you start Emacs (except when you specify `-q').
But your init file, if any, is loaded first; if it sets
inhibit-default-init non-nil, then `default' is not
loaded.
Your site may also have a site startup file; this is named `site-start.el', if it exists. Emacs loads this library before it loads your init file. To inhibit loading of this library, use the option `-no-site-file'.
If you have a large amount of code in your `.emacs' file, you
should move it into another file such as `~/something.el',
byte-compile it, and make your `.emacs' file load it with
(load "~/something"). See section `Byte Compilation' in the Emacs Lisp Reference Manual, for more
information about compiling Emacs Lisp programs.