The No Dealing Desk or NDD type of forex broker lacks a dealing desk, as the name implies. Nevertheless, it instead offers the best composite quotes obtained from the multiple forex quote providers that give the NDD broker its liquidity in the forex market.
Typically, each of these providers will post their best markets with the NDD forex broker. The broker then executes transactions and watches and fills orders for their clients accordingly. The NDD forex broker generally either charges a commission or they widen the bid/offer spread in order to make a small profit on each trade executed, sometimes they do both.