Stock exchange

A stock exchange is a form of exchange which provides services for stock brokers and traders to buy or sell stocks, bonds, and other securities. Stock exchanges also provide facilities for issue and redemption of securities and other financial instruments, and capital events including the payment of income and dividends.[1]

Stock exchanges make money through the following:

1. Having a listing fee on the companies that are traded on the exchange. The fee can be as low as $25K, but can go as high as $250K/year per stock listed on an exchange. Multiply that by 3000-4000 odd companies in New York Stock Exchange (NYSE) and you will get an idea of how big the revenue is. Add to that all the 1000s of Mutual funds and 100s of ETFs listed on the exchange.
2. Data fee. The real time data they give to companies such as Bloomberg and to online services such as Google Finance is huge. This revenue stream can be as high as $100 million year per exchange.
3. Trading software and technology sales to institutional investors such as funds. The trading software ensures that the clients get the data faster than other market participants.
4. Fees on trading (their traditional source). Every time you buy or sell a stock you pay a small fee to the exchange (paid per 100 stocks). This fee can be high for derivatives and other exotic trading instruments. This fee is typically 1/300th of a dollar per share for stocks. [2]

Reference

1. Stock exchange from Wikipedia
   
2. How do stock exchanges (such as NYSE) make money? From Quora