Most FX brokers are Market Makers (operating Dealing Desks) who "make" both the bid and the ask prices on their trading systems and display them publicly on their quote screens. They undertake to execute transactions at the prices they have quoted to their customers. As counterparties to each FX transaction Market Makers must therefore take the opposite side of a customers trade. So whenever a trader sells, they must buy from them, and vice versa. This traditional DD trading model is the most commonly practicised throughout the Retail FX trading marketplace.
ECN platforms on the other hand pass on prices from multiple market participants, such as banks and Market Makers, as well as other traders connected to the ECN, and display the best bid/ask quotes on their trading platforms based on these prices. ECN platform providers make money by charging customers a fixed commission for each transaction. Authentic ECNs do not play any role in making or setting prices. This means the risks of price manipulation are reduced for retail traders. Once again ECN-style brokers serve as counterparties to FX transactions, but they operate on a settlement rather than pricing basis. Unlike fixed spreads, which are offered by some Market Makers, spreads of currency pairs vary on ECNs depending on the pair's trading activities.