Warehouse is a place where you can store items and gold. It can be accessed through NPCs in towns.
By default every player has one page slot to store items. But it is possible to unlock additional pages through the Warehouse Expansion System.
In this system you expand your Warehouse by doing quests.
This quests are called "Warehouse Expansion", and can be found in the Quest Book by pressing the key "Q".
Every completed quest unlock one additional slot.
There is 4 additional slots that can be unlocked completing quests.
You cannot store potions on warehouse.