An ECN broker is a broker that has connections to liquidity providers and gives their clients direct access to the interbank system. This differs from market maker brokers who become the counterparty to all trades made with them. Also unlike with market makers, the spreads will be variable while some marker makers offer fixed spreads. But since you have access to several liquidity providers, you will typically get much better spreads than you would with a market maker. Also, since there is no intervention, trades are executed immediately.