To use it, type M-x list-packages in Emacs.
(This requires Emacs version 24.1 or higher.)
Since you'll probably want to use your installed packages, it's also
recommended
that you add (package-initialize) somewhere
in your ~/.emacs file.
Packages are managed through the GNU ELPA repository (see the GNU Emacs project page for clone instructions).
To contribute new packages refer to the README.