Dealer: Sets the Bid/Ask price as he sees fit, possibly based on market conditions, possibly not. Paid by the spread that he decides to set.
ECN: Supposedly matches orders between traders. Does not modify prices, just posts orders from all sources. Traders typically include other retail traders and "liquidity providers". Think of it as a mini-exchange. This is the Broker model, and is compensated primarily by comission.
STP: Supposedly gives retail trader access to best price of participating liquidity poviders without intervention. Could be compensated by cut of spread or commission.
You have to be a KID to believe in this bullshit. You are always trading against a dealer, and it doesn't matter via ECN or not.
Just open your eyes ! It's just another marketing tool. Traders are loosing money not because the dealers on the other side.
