HFT is a technology category rather than a single strategy. The recognisable families are electronic market making, latency arbitrage between correlated instruments or venues, statistical arbitrage over seconds to minutes, and rebate-driven passive strategies.
What they share is economics: tiny expected value per trade multiplied by enormous trade counts, which only works if latency, fees and adverse-selection are all controlled tightly.
Example: a firm nets $0.0006 per share on 90 million shares a day, about $54,000 daily gross. A single day of being systematically picked off at $0.004 per share on 10 million shares wipes out $40,000 of that — which is why risk controls and feed quality matter more than raw speed alone.
Related: electronic-market-making, latency-arbitrage, colocation, toxic-flow