ECN stands for Electronic Communications Network. Due to the Forex market being largely unregulated, there is no central exchange for Forex and trading is performed on an OTC (Over The Counter) market. ECN trading gives you direct access to the Forex market (interbank market) where you can trade with other traders and your orders are actually displayed in the market and are seen by others, who in turn can introduce their own orders and if the prices match, a deal is complete.
ECN is the next step in Forex trading bringing the Forex market closer to the stock market model and is the route all brokers will take one day. Right now, FXOpen is the first broker who successfully brought you the ECN trading model in the familiar and preferred MetaTrader (MT4) interface.