Gringotts is the wizards' bank--their only one. It's run by goblins, and is considered almost the safest place for a wizard to keep anything. The vaults are located hundreds of miles below London, guarded by dragons, and hard to find one's way around.
Access to the vaults can be had through their main building on Diagon Alley. It is a snowy-white building that towers over most of the shops on the street. Its front doors are made of bronze; behind them are silver doors with a poem about greed inscribed on it. Inside is a vast marble hall with close to a hundred goblins sitting behind a long counter, and many doors.
Some of the doors lead to torchlit stone passageways with railroad tracks, the main method of accessing the vaults. The carts that ride on the tracks travel at enormous speeds down to the vaults, through caverns with underground lakes. Some of the vaults have keyholes, while others can only be opened by Gringotts employees--all others would be sucked inside the vault.