We have known for a while that the new Fire Emblem would be on two alternate versions, but Nintendo today announced more details about how it will all work.
Birthright and Conquest will tell two versions of the same story. Players only really need one version to experience the game, but if they decide to buy one of the versions for $40 they will be able to download the other version of the game for $20, making it a more reasonable option.
Beyond these two versions, there will also be epilogue chapter called Revelation and will be released after both Birthright and Conquest. There will also be a collectors edition that will include all chapters in the game for $80, and will also include an art book and pouch for the 3DS.
There will also be additional content after the game's launch. The game will be coming out on Feb 19, 2016.