What is an STP broker?
Some brokers claim that they are true ECN brokers, but in reality, they merely have a Straight Through Processing system.
Forex brokers that have an STP system route the orders of their clients directly to their liquidity providers who have access to the interbank market. NDD STP brokers usually have many liquidity providers, with each provider quoting its own bid and ask price.
Let's say your NDD STP broker has three different liquidity providers. In their system, they will see three different pairs of bid and ask quotes.
|Liquidity Provider A||1.2998||1.3001|
|Liquidity Provider B||1.2999||1.3001|
|Liquidity Provider C||1.3000||1.3002|
Their system then sorts these bid and ask quotes from best to worst. In this case, the best price in the bid side is 1.3000 (you want to sell high) and the best price on the ask side is 1.3001 (you want to buy low). The bid/ask is now 1.3000/1.3001.
Will this be the quote that you will see on your platform?
Of course not!
Your broker isn't running a charity! Your broker didn't go through all that trouble of sorting through those quotes for free!
To compensate them for their trouble, your broker adds a small, usually fixed, markup. If their policy is to add a 1-pip mark-up, the quote you will see on your platform would be 1.2999/1.3002. You will see a 3-pip spread. The 1-pip spread turns into a 3-pip spread for you.
So when you decide to buy 100,000 units of EUR/USD at 1.3002, your order is sent through your broker and then routed to either Liquidity Provider A or B.
If your order is acknowledged, Liquidity Provider A or B will have a short position of 100,000 units of EUR/USD 1.3001, and you will have a long position of 100,000 units of EUR/USD at 1.3002. Your broker will earn 1 pip in revenue.
This changing bid/ask quote is also the reason why most STP type brokers have variable spreads. If the spreads of their liquidity providers widen, they have no choice but to widen their spreads too. While some STP brokers do offer fixed spreads, most have VARIABLE spreads.
What is an ECN Broker?
True ECN brokers, on the other hand, allow the orders of their clients to interact with the orders of other participants in the ECN.
Participants could be banks, retail traders, hedge funds, and even other brokers. In essence, participants trade against each other by offering their best bid and ask prices.
ECNs also allow their clients to see the "Depth of Market." Depth of Market displays where the buy and sell orders of other market participants are. Because of the nature ECN, it is very difficult to slap on a fixed mark-up so ECN brokers usually get compensated through a small COMMISSION.