The red herring is as much a debate tactic as it is a logical fallacy. It is a fallacy of distraction, and is committed when a listener attempts to divert an arguer from his argument by introducing another topic. This can be one of the most frustrating, and effective, fallacies to observe.
The fallacy gets its name from fox hunting, specifically from the practice of using smoked herrings, which are red, to distract hounds from the scent of their quarry. Just as a hound may be prevented from catching a fox by distracting it with a red herring, so an arguer may be prevented from proving his point by distracting him with a tangential issue.
Example
Many of the fallacies of relevance can take red herring form. An appeal to pity, for example, can be used to distract from the issue at hand:
“You may think that he cheated on the test, but look at the poor little thing! How would he feel if you made him sit it again?”