STP stands for Straight Through Processing and is the process that some brokers use to remit the orders straight to their liquidity providers. An STP brokers could have more than one liquidity provider and it will be able to process order in a quick manner but STP brokers make money on spreads, thus even though they do not have a physical dealing desk to monitor and counter-trade client orders.
STP forex brokers are also interested to see their clients trading profitable, so that a broker can continue earning on spreads.
ECN stand for Electronic Currency Transfer. It is view in the industry as a more fair way of facilitating CFD and Forex trader a fairer trading experience without the need for brokers to be the middle man. ECN brokers providers a marketplace where all orders interact with other clients orders in the open market, like this the market remain competive with sell and buy offers in real time.
ECN brokers profit from commission when clients submit theirs orders. Therefor ECN brokers are interested that client keep winning so they don’t run out of capital to trade.
Dealing Desk brokers are typically those who have a physical trading desk with actual traders trading in financial products, mainly banks and financial institution like hedge fund or investment banks. Some are hybrid system like including STP or ECN facilities.
In general this types of broker profit from commission, wide spread and offset the amount off liquidity that they could be exposed to. Meaning that in most cases they will hedge against their client position.