Backpacks are single use (They are consumed when used on a character).
Backpacks provide 8 extra inventory slots which can be switched to via a tab next to the stats tab.
Backpacks are not permanent. When you die, you lose them and their contents with the character.
Backpacks are only obtained with Realm Gold, which can only be obtained by donating. (With the new Kabam Rewards program it is possible to get Realm Gold for free)
Backpacks are soulbound (They are not trade-compatible).