Quote from beachhouse:
See=> that's a computer software matter. When an order is initiated, it has to be entered into the system, from your computer to the exchange. It travels for a distance, just like you walk from your home to the office.
When you walk out of your home, people can see you.
When you walk on the sidewalk, people can see you.
When you approach your office, people can see you.
When your order is logged into your computer, another computer can see you.
When your order goes through your broker's server, another computer can see you.
When your order goes from the broker's server to the exchange server, another computer can see you. (here is a fraud often committed by brokers: your order is internalized, never sent to the exchange by the broker)
Before your order reaches the exchange server, another computer can see you.
When I see you want to buy 1000 shares, I will buy BEFORE your order reaches the exchange, lifting the asking price by 1 cent or 2. When your order arrives, I sell you what I have just bought at the current asking price (remember I just raised the asking price), making some no-risk money.
If I keep doing that all day, I make a lot of money. That's why Goldman Sucks never lose in "trading." (front-running actually).