The directory ~/.emacs.d/ is a standard location for additional per-user Emacs-specific files. Various packages store information in this directory. Since it is located in the home directory (~/), it is unique for each user, just like the ~/.emacs init file. Note that as .emacs.d begins with a dot, it is normally hidden on Unix systems and only turns up in directory listings if all files and folders are shown (ls -a).

Starting with Emacs 22, if ~/.emacs does not exist, Emacs will try ~/.emacs.d/init.el and ~/.emacs.d/init.elc. You can also put the shell init file .emacs_SHELL under ~/.emacs.d.

Starting with Emacs 23, the variable user-emacs-directory holds the location of .emacs.d. On Windows, the location depends on the HOME environment variable: if set to C:\, then .emacs is located at C:\.emacs and .emacs.d at C:\.emacs.d. On Windows 7 the default is C:\Users\epich\AppData?\Roaming\.emacs and .emacs.d .

The user directory itself is not part of load-path, and since it contains other things than libraries, you should leave it at that. Instead, you should install packages in a dedicated sub-directory (e.g., ~/.emacs.d/elisp) and add that to the load-path instead.

See also DotEmacsBankruptcy.