A market maker is a company, or an individual, that quotes both a buy and a sell price in a financial instrument or commodity held in inventory, hoping to make a profit on the bid-offer spread, or turn. From a market microstructure theory standpoint, market makers are net sellers of an option to be adversely selected at a premium proportional to the trading range at which they are willing to provide liquidity.
Most foreign exchange trading firms are market maker and so are many banks, although not in all currency markets. In foreign exchange (or FX) trading, where most deals are conducted over-the-counter and are, therefore, completely virtual, the market maker sells to and buys from its clients and is compensated by means of price differentials for the service of providing liquidity, reducing transaction costs and facilitating trade. Recent developments in the over-the-counter FX market have permitted even buy-side (non bank participants) virtually to act as market-makers through the advent of high speed/frequency software engines that submit bids and offers outside prices available on other networks or ECN (electronic communication network) where FX is traded.
How do Market Makers make their Money?
Market Makers must be compensated for the risk they take; what if he buys your shares in IBM then IBM's stock price begins to fall before a willing buyer has purchased the shares? To prevent this, the market maker maintains a spread on each stock he covers. Using our previous example, the market maker may purchase your shares of IBM from you for $100 each (the ask price) and then offer to sell them to a buyer at $100.05 (bid). The difference between the ask and bid price is only $.05, but by trading millions of shares a day, he's managed to pocket a significant chunk of change to offset his risk.