There are three units from three retailers, check here: http://www.totalwar.com/empire/gameinfo/pre_order.php


All of the units are described here:
http://www.totalwar.com/empire/gamei....php#aanswer_4

The Special Forces Edition has six special units as described.

Each of the first three retailers on the list have an exclusive unit they provide if you order from them.

Amazon.com: Dahomey Amazons

Play.com: USS Constitution

Game: Death’s Head Hussars


That’s it in a nut shell.