Just to add on to this guy's explanation of what High Frequency Trading (HFT) is: There is another kind of HFT trading which is structurally unfair. (There might have been a video on this here at one point.)

Basically exchanges offer services to traders. One such service is a heads up on incoming orders. For instance, if A wants to buy 100,000 shares of MMM at $50 a share, usually not all 100,000 shares will be acquired at $50 each. What will happen is the first 40k shares are at $50 each, then the subsequent 60,000 shares are purchased at higher prices per share.

This happens in the following manner: A puts in an order for 100,000 shares of MMM at $50 each. That 100,000 order gets blindly routed to a given exchange to be fulfilled. Call this Exchange 1. Exchange 1 sells a heads up service on orders. In this case it tells its clients that it has an order for 100,000 shares of MMM at $50 each and it will fulfill 40,000 shares of that order. The clients of Exchange 1 get that information, then within microseconds, place orders at the next five exchanges in proximity for orders of MMM at $50. This causes MMM to trade at a price higher than $50 at Exchange 2, 3, 4, 5, etc., say $52 a share.

A's remaining order is sent to Exchange 2 and fulfilled at $52 a share. Then on to Exchange 3, and so on. Then the client can immediately sell at $52 or whatever price at a $2 gain per share.

This is the kind of HFT which is really unfair and unreasonable and it is structural because Exchange 1 is designed to allow for heads up views of orders before they are fulfilled.

Thats interesting but I think we could improve it even further by firing everyone involved in 'finance' out of a giant cannon into space.