HowStuffWorks "Quote Services, Interfaces and Matching Engines":
"Of quote services, interfaces and matching engines, quote services are perhaps the easiest to understand. Every second of every day, the prices of stocks are fluctuating. And people around the world want to see those prices in real time: Broker dealers want to provide quotes to their customers, and news organizations want quotes to display in their shows. To meet this need, NASDAQ provides up-to-the-minute price quotes through its computers, which can see what's happening inside the matching engine and then send that data out to the rest of the world."
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