Download
The best way to obtain AUCTeX is by using the Emacs package manager integrated in Emacs 24 and greater (GNU ELPA). Simply do M-x list-packages RET, mark the auctex package for installation with i, and hit x to execute the installation procedure. That's all.
This installation procedure has several advantages. Besides being platform and OS independent, you will receive intermediate releases between major AUCTeX releases conveniently.