But as new marketplaces have emerged, PCs have been unable to compete with Wall Street’s computers. Powerful algorithms  “algos,” in industry parlance  execute millions of orders a second and scan dozens of public and private marketplaces simultaneously. They can spot trends before other investors can blink, changing orders and strategies within milliseconds.

High-frequency traders often confound other investors by issuing and then canceling orders almost simultaneously. Loopholes in market rules give high-speed investors an early glance at how others are trading. And their computers can essentially bully slower investors into giving up profits  and then disappear before anyone even knows they were there.

High-frequency traders also benefit from competition among the various exchanges, which pay small fees that are often collected by the biggest and most active traders  typically a quarter of a cent per share to whoever arrives first. Those small payments, spread over millions of shares, help high-speed investors profit simply by trading enormous numbers of shares, even if they buy or sell at a modest loss.

“It’s become a technological arms race, and what separates winners and losers is how fast they can move,” said Joseph M. Mecane of NYSE Euronext, which operates the New York Stock Exchange. “Markets need liquidity, and high-frequency traders provide opportunities for other investors to buy and sell.”

The rise of high-frequency trading helps explain why activity on the nation’s stock exchanges has exploded. Average daily volume has soared by 164 percent since 2005, according to data from NYSE. Although precise figures are elusive, stock exchanges say that a handful of high-frequency traders now account for a more than half of all trades. To understand this high-speed world, consider what happened when slow-moving traders went up against high-frequency robots earlier this month, and ended up handing spoils to lightning-fast computers.

It was July 15, and Intel, the computer chip giant, had reporting robust earnings the night before. Some investors, smelling opportunity, set out to buy shares in the semiconductor company Broadcom. (Their activities were described by an investor at a major Wall Street firm who spoke on the condition of anonymity to protect his job.) The slower traders faced a quandary: If they sought to buy a large number of shares at once, they would tip their hand and risk driving up Broadcom’s price. So, as is often the case on Wall Street, they divided their orders into dozens of small batches, hoping to cover their tracks. One second after the market opened, shares of Broadcom started changing hands at $26.20.

The slower traders began issuing buy orders. But rather than being shown to all potential sellers at the same time, some of those orders were most likely routed to a collection of high-frequency traders for just 30 milliseconds  0.03 seconds  in what are known as flash orders. While markets are supposed to ensure transparency by showing orders to everyone simultaneously, a loophole in regulations allows marketplaces like Nasdaq to show traders some orders ahead of everyone else in exchange for a fee.