Pokémon Bank is an application and service for Pokémon X and Pokémon Y that will allow you to deposit, store, and manage your Pokémon in private Boxes on the Internet! Pokémon Bank is a paid service, with an annual charge for usage.
According to www.PokemonXY.com, Pokemon Bank can hold up to 3,000 Pokemon!
Also, coming with Pokemon Bank is another service called Poke Transfer.
When you download Pokémon Bank, you can also download the linked application Poké Transporter! When you insert a Pokémon Black Version, Pokémon White Version, Pokémon Black Version 2, or Pokémon White Version 2 Game Card into a Nintendo 3DS system, you'll be able to use Poké Transporter to transfer Pokémon from these games into their own online Boxes. Then, you can easily transfer those Pokémon into Pokémon X and Pokémon Y usingPokémon Bank!
Poke Transporter will be a free service, while Pokemon Bank will come with a free trial period when it releases.
For more information, visit http://www.pokemonxy.com