What is algorithmic or program trading?

While “program trading” generally refers to computerized trading for a group of securities, and “algorithmic trading” refers to trading decisions made by computers using mathematical equations, in fact both are nearly synonymous today. In both cases, trading decisions incorporate instructions about price ranges, volume, comparative behaviors, the reactions of other participants, the costs and implications of trading decisions on the market and the portfolio of securities for which the trades are being executed, and macroeconomic data related to overall risk. Often, large or specialty broker-dealers execute these trades on behalf of their institutional trading clients, which may include rational/fundamental investors, risk managers and speculators.