DOM price ladder display allow traders to follow very liquid securities and quickly enter/exit the market at exact levels. It is often used to trade futures or forex, but can be used to trade stocks as well.
Here is what the DOM display looks like in Medved Trader:
Note that there are two connections on this window, one (on Main tab) is for the market data, the other (on Trading tab) is for the trading broker connection. The two connections do not have to be the same.