(or 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, ifor doesn't exists, Emacs will look for and .
For versions 23 or superior, the variablecontains the location of .
Since Emacs 24, Package.el downloaded packages are stored there, in a subfolder named ELPA.