A seed bank is a secure, climate controlled collection of seeds that are saved in case of annihilation of all plants, at least that is how I currently understand it,
There are two international seed banks. These are a back-up for National Seed Banks.
Most countries have a National Seed Bank. Unfortunately, these can be, and have been, destroyed by natural disasters or wars. Recently, one of the international seed banks sent Syrian seeds to other countries that are not currently affected by war; so that there is a regional source and so that seed scientists can continue to grow, test and collect the seed.
The Svalbard Global Seed Vault is buried in a mountain on an inland in Norway, close to the Arctic Circle. This TED (Technology, Education, Design) Talk is a good place to start.
The Millennium Seed Bank Partnership in London has a slightly different philosophy. This TED Talk is an excellent explanation.
Can you identify the difference in the philosophy behind the two international seed banks?