Great piece in Smithsonian.com which covers work done by Eric Hunsader at NANEX.
NANEX produced a great video which shows one half second of JNJ order flow. It was slowed down so we can visualize the flow and see fast the top of book changes at each exchanges. Make sure to watch in HD and full-screen to improve the experience:
According to Eric Hunsader:
Each box represents one exchange. The SIP (CQS in this case) is the box at 6 o’clock. It shows the National Best Bid/Offer. Watch how much it changes in a fraction of a second. The shapes represent quote changes which are the result of a change to the top of the book at each exchange. The time at the bottom of the screen is Eastern Time HH:MM:SS:mmm (mmm = millisecond). We slow time down so you can see what goes on at the millisecond level. A millisecond (ms) is 1/1000th of a second.
This video was part of Mr. Hunsader presentation at the Wired conference.