I am only relatively new to Foreign Exchange also but from what I understand it's like every other derivative/instrument in that the broker'makes a market'.
In other words the broker likes to have a buyer for each seller and he makes his money from the spread on the transaction.
It is quite a lot more complex than that (they have the power to move the market for various reasons that you will shortly become conscious of) when they have enough capital to fulfill the orders.
Basically they run a book, like a level two display which includes both buy and sell orders and...