In case you don't want to read the whole thing, although I highly recommend it, here's some meat:

As they worked through the order types, the Puzzle Masters created a taxonomy of predatory behavior in the stock market. Broadly speaking, it appeared as if there were three activities that led to a vast amount of grotesquely unfair trading. The first they called electronic front-running — seeing an investor trying to do something in one place and racing ahead of him to the next (what had happened to Katsuyama when he traded at RBC). The second they called rebate arbitrage — using the new complexity to game the seizing of whatever legal kickbacks, called rebates within the industry, the exchange offered without actually providing the liquidity that the rebate was presumably meant to entice. The third, and probably by far the most widespread, they called slow-market arbitrage. This occurred when a high-frequency trader was able to see the price of a stock change on one exchange and pick off orders sitting on other exchanges before those exchanges were able to react. This happened all day, every day, and very likely generated more billions of dollars a year than the other strategies combined.