.emacs.d

${HOME}/.emacs.d (or %HOME%\.emacs.d on Windows) is the standard location for additional Emacs Lisp files. Many packages uses it to store information, such as configuration files. It's unique for each user, as it's located inside the personal home directory. It is a "dotfolder" (starts with a dot), so it is normally hidden on most systems.

Since Emacs 22, if ${HOME}/.emacs or ${HOME}/.emacs doesn't exists, Emacs will look for ${HOME}/.emacs.d/init.el and ${HOME}/.emacs.d/init.elc.

For versions 23 or superior, the variable user-emacs-directory contains the location of .emacs.d.

Since Emacs 24, Package.el downloaded packages are stored there, in a subfolder named ELPA.