Bank

Overview
Warehouses, aka Banks, are very useful NPCs. They allow you to store any item that can go in your bags.

Where can I find them?
Warehouses can be found on the map by their icon- a lockbox.

They are also often found in hubs with portals to Mirage Island and Auction Houses nearby.

Storage
Any item that can be stored in a bag can be stored in a warehouse. A warehouse by default has 50 slots but can be upgraded 10 slots at a time for an increasing amount of gold for up to 6 slots.

P.S. Starting with the 1.2 patch does not improve for the gold.