Vault
Storage that belongs to the account, not to the character: everything below is shared by every character on your key.
Items
The item vault has no size limit. Items go in and come out through a store window, a page at a time, and an item the trade post could not sell comes back here rather than to your pack.
It belongs to the account, not the character, which makes it the way to move things between your own characters: put it in with one, take it out with another. It saves the moment an item lands in it - if the server fell over immediately afterwards the item would still be there when you came back. Identical items stack, and only identical ones: same name, same appearance, same properties.
Crafting draws on it directly. You do not have to take the materials out first.
What it will not take: a player corpse, some plot items, the wings rune - and anything taken with Pick Pocket while it still counts as stolen.
Opening it

From the Player Menu, or by typing:
/vault | everything |
/vault search "sword" | opens with that already in the box |
/vault last 10 | the ten most recently added |
Searching it is more than a name box. The search understands:
| You type | It finds |
|---|---|
sword | anything whose name or item type contains "sword" |
"flaming longsword" | that exact phrase |
sword steel | both words - so do AND and && |
sword OR axe | either |
-cursed | everything except - so do NOT and ! |
(armor OR shield) ilr>5 | brackets group, and conditions combine |
ilr>10 | items above that item level; < and = work too |
sword last 5 | the five most recently stored among the matches |
ilr and category are the only two conditions that exist; anything else after a > or = matches nothing. An empty search returns everything.
The categories are numbered, and are the same tabs an ordinary merchant shows:
category=0 | armour |
category=1 | weapons |
category=2 | potions and scrolls |
category=3 | jewellery, wands and magic staves |
category=4 | everything else |
So category=0 ilr>10 is armour above item level 10, and category=3 !crafted is jewellery and wands that did not come out of GTS.
A Portable Banker costs 1,000,000 gold and belongs to the character that bought it. Once bought, deposit and withdraw appear inside your own player merchant, so you never have to walk to a bank again.

Gold
The account has a gold balance of its own. It is what a deleted character's gold is paid into, and it is spent as a second purse: when a resurrection costs more gold than you are carrying, the rest comes out of the bank.
Experience
Experience can be banked too, and the terms are worth reading before you use it:
- A deposit is charged 25%. You lose the experience you deposit, and a quarter of it is kept.
- A withdrawal only tops you up to the next level: you cannot withdraw while you are between levels, and never past the threshold above you.
- You cannot deposit or withdraw while polymorphed.
- A hardcore character is refused a withdrawal, and a deposit stores nothing.
- A premium subrace must be paid for before that character can withdraw anything.
Deleting a character banks its experience to the account, and anything above what level 40 needs - 780,000 - comes back as Epic tokens instead.