Here you can read the difference between the different Forex Broker types:
DD/MM = Dealing Desk / Market Maker
These Forex Brokers route orders through a Dealing Desk and quote fixed spreads. Dealing Desk Brokers make money while trade against clients and with the spread. They also called Market Makers, because they always trade the opposite of the client. Traders don’t see real market quotes because they manipulate quotes.
NDD = No Dealing Desk
No Dealing Desk Forex Brokers provide access directly to the interbank market. There are no requotes, order restrictions, delays or pausing order confirmation. This allows trading during news times. No Dealing Desk Brokers can charge commissions or increase the spreads.
ECN = Electronic Communication Network
ECN Forex Broker acts like a marketplace, where all clients, banks, market makers and traders trade against each other by sending competing offers and bids. Clients get the best offers for their trades, available at that time because all orders are matched between counter parties in real time.
STP = Straight Through Processing
STP Forex Brokers send client orders directly to the liquidity provider, for examples Interbanks. STP Forex Brokers can have multiply liquidity providers, more providers are better because the fills for the client is better and faster. There is no dealer intervention any all trades pass in the real market.