Ran across this good explanation of the “high-frequency trading” term thrown around a lot: Algorithmic Trading is Not High Frequency Trading.  The impact on the markets of HFT is very different compared to having computers aid in the execution of a human decision.

Even when a human enters a simple order on a stock brokerage application like “buy 100 Apple shares”, there’s computer logic being used on their behalf.  Depending if you asked for a “market” order (any price), or set a limit price, a computer is acting on your behalf to respect those rules and get you the best price possible.