As you may know, Game Freak revealed a service with Pokémon X and Y that allows players to store and retrieve their Pokémon from a cloud service in the internet. This allows Pokémon to be easily transferred to and from Pokémon X and Y in bulk, without having to have two systems and trade. The Bank app also has advanced sorting and filtering features to easily sort through up to 3000 Pokémon from many different Pokémon games.
Nintendo today have confirmed Bank's release date on the 3DS eShop: December 27th. They've also launched a special site for it with lots more information, which can be found here, and released quite a nice trailer showing it and Poké Transporter off.
Finally, pricing has been confirmed: $4.99 USD / €4,99 EUR / £4.49 GBP per year. If you first download Bank before February 1st, 2014, you will be issued with a pass to use the service for free for 30 days, starting when you first launch Pokémon Bank.
Bank will also be released with a companion app called Poké Transporter which allows you to transfer Pokémon from Pokémon Black, White, Black 2, or White 2 into Bank, which then can be downloaded into Pokémon X and Y. It will be free to download from the eShop and use your Bank subscription to function.