A red herring is an argument which is used to distract from the original issue by introducing some irrelevant issue or topic. This seems to be thought of as both a logical fallacy and a debating tactic. I guess it is a rhetorical tool when used consciously but is always a fallacy. Its basic structure is; 1) A is the topic of discussion, 2) B is introduced as a distraction from topic A, 3) A is abandoned. Supposedly, the etymology of this fallacy came from the training of dogs in the sport of fox hunting. The dog trainer would drag a cured red herring across the trail of a fox to confuse the dogs. Eventually the dogs would be able to follow the scent of the fox rather than the scent of the herring.
An example might be; "This new computer program is too slow for me to be able to get my work done on time and it is missing some of the necessary features the old program used to have." Steve replies "What you have failed to understand is that the new software cost half as much as the other program and is much easier to learn." With this example Steve's argument does not in anyway address the original topic which is that the program is too slow and missing necessary features. Instead he introduces an irrelevant argument regarding costs and learning time which distracts from the original issue. This is not to say that Steve's argument is irrelevant to some other issues but it does not in anyway address the original issue.