The pre-installed Emacs that ship with OS X are built without a GUI. Hence,
$ emacs
Will NOT open a graphics window and instead open a text / terminal version instead. Please note that this version (installed in /usr/bin/emacs) is also old and is at level 22.1.1 in Mountain Lion.
You can download the latest Emacs build to get the desired behavior (and also get the latest version of Emacs as a bonus). It is available in various places, including http://emacsforosx.com/ .
Most of these pre-installed Emacs are installed under /Applications, and to call from the terminal you will need to specify the full path to the binary file, which is usually equal to:
/Applications/Emacs.app/Contents/MacOS/Emacs
.bashrc :
alias emacs=/Applications/Emacs.app/Contents/MacOS/Emacs
emacs .