I got this from a good book, whose title...
Automated Trading Systems are Event-Driven Applications (EDAs). Unlike traditional applications, which follow their own program flow, event-driven programs react to external events, in our case to events that occur on electronic exchanges, namely changes in bid and ask prices and ladder volumes and trades. EDAs are programmed with event loops, to look repeatedly for new information and then execute a trigger function. As a result, programming a trading system is a matter of writing the trigger functions that gather the new information and make trading decisions. These trigger functions are called event handlers. The CLR controls a dispatcher, which will call the event handlers using a queue to hold unprocessed events.
