Front running is the illegal practice of a stock broker executing orders on a security for its own account while taking advantage of advance knowledge of pending orders from its customers. When orders previously submitted by its customers will predictably affect the price of the security, purchasing first for its own account gives the broker an unfair advantage, since it can expect to close out its position at a profit based on the new price level. Front running may involve either buying (where the broker buys for their account, before filling customer buy orders that drive up the price) or selling (where the broker sells for its own account, before filling customer sell orders that drive down the price).

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A practice similar to front running is called "tailgating". Tailgating means the action of a broker or adviser purchasing or selling a security for his or her client(s) and then immediately making the same transaction in his or her own account. This is not illegal like front running, but it is not looked upon favorably because the broker is most likely placing a trade for his or her own account based on what the client knows (like inside information).

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DKos Diary Reverberates Throughout Wall St. (w/update)

Interestingly, it is also possible that, simply due to the enhanced speed (as mentioned in numerous pieces about this story) with which one would be enabled to follow/mimic another trade at Goldman-Sachs (with the assumption being that the tailgating trade was executed on a faster "channel" or system, or modulated with a higher-priority tag on it, than the original trade), after it commenced, one could then conclude their "tailgating" trade in a more timely manner than the original trade, if it was facilitated on a slower system and/or modulated on a "lower-priority channel." (For instance, routing instructions for the tailgating trade might be made via a "priority path" to the New York Stock Exchange, while the original trade might be tagged to be executed on a "standard" and/or "normal path.")