A vertical display of resting order quantities at each price level, plus the order-entry controls that let a trader click directly into the book.
A market depth chart. Each step adds the orders resting at the next price away from the middle, so the bars show how much can be bought or sold before the price has to move. Here the buy side is the taller wall.
The DOM is the native interface of futures trading: prices run down the centre, bid size on one side, ask size on the other, working orders and position marked on the ladder itself. It is level-2 data made clickable.
Its value is showing where size sits rather than where price has been. Its limitation is that it shows only displayed quantity — iceberg-orders, hidden-orders and off-book interest are invisible.
Example: the ladder shows 850 contracts bid at 5,000.00 and 90 offered at 5,000.25. That looks like a wall, but if 400 of the bid is refreshing iceberg size and another 300 cancels the moment price approaches, the real defended quantity may be under 200.