DD means a Dealing Desk broker who acts as a market maker (i.e. accepts orders to buy or sell in order to provide a liquid market). Because Dealing Desk brokers have to accept orders and are exposed to customer trades (whether winners or losers) they may also hedge trades by taking the opposite position to them.
NDD means No Dealing Desk. Orders are placed directly into the interbank market. If there is match between a buyer and seller of a currency pair, the order is filled.
STP means Straight Through Processing. Trades are automatically immediately processed on the interbank market without broker intervention.
ECN means Electronic Communications Network. ECN brokers display an electronic order book of buyers and sellers which traders can place their orders against.
External LinksTypes of forex brokers: ECN - STP - NDD - DD
Forex Central article on types of brokers.How do I choose a broker: ECN / STP or Market Maker?
Forex Central article describing the pros and cons of different brokers.The Essentials of Retail Forex Broker Models | How They Work and Finding the Good Guys
In-depth article from NoBrainerTraders.com about Forex brokers and how to choose between them.