As has been pointed out a few times in connection with this incident, Knight is largely a market-maker (not an HFT firm): Knight keeps open quotes on both buy and sell sides of transactions at all times and provides liquidity. Their strategy to make money is the "spread" -- they price the buy a couple of cents lower than the sell and on average they end up buying a couple cents lower than selling.

They are part of the infrastructure of the major markets; a company listing on NASDAQ or NYSE is required by the market to choose at least one market-maker who will be designated to always keep open buy and sell orders on the books.

As it happens, Knight does its market-making through computers; this replaces sweaty men who would stand in the "pit" of the trading floor and trade by waving fingers at each other (including, occasionally, the middle one). It looks like HFT in that computers trade quickly, but it is intended to be a liquidity-providing service to the market: there is always somebody willing to buy or sell anytime the market is open.

(In contrast, HFT does not require always-open buy and sell quotes.)