Argumentum ad verecundiam (argument or appeal to authority). This fallacy occurs when someone tries to demonstrate the truth of a proposition by citing some person who agrees, even though that person may have no expertise in the given area. For instance, some people like to quote Einstein's opinions about politics (he tended to have fairly left-wing views), as though Einstein were a political philosopher rather than a physicist. Of course, it is not a fallacy at all to rely on authorities whose expertise relates to the question at hand, especially with regard to questions of fact that could not easily be answered by a layman -- for instance, it makes perfect sense to quote Stephen Hawking on the subject of black holes.
At least in some forms of debate, quoting various sources to support one's position is not just acceptable but mandatory. In general, there is nothing wrong with doing so. Even if the person quoted has no particular expertise in the area, he may have had a particularly eloquent way of saying something that makes for a more persuasive speech. In general, debaters should be called down for committing argumentum ad verecundiam only when (a) they rely on an unqualified source for information about facts without other (qualified) sources of verification, or (b) they imply that some policy must be right simply because so-and-so thought so.
http://www.csun.edu/~dgw61315/fallacies.html
http://en.wikipedia.org/wiki/Appeal_to_authority
Argumentum ad vericundiam
This is a move in argument that may or may not be fallacious, depending on the circumstances. It means an appeal to authority, an example of which could be thus:
You say philosophy is important, but Professor X says it's a waste of time.
Here the speaker refers to the authority of the professor to counter the claim that philosophy is important. The problem is that the presumed authority may or may not be relevant: if the professor is (or was) a lifelong student of philosophy and decided after years working in the field that it really is a waste of time, then perhaps we should look into his reasons for saying so? On the other hand, if he is a professor of mineralogy, say, then—on the face of it—his opinion bears no more or less weight than anyone else's. It may be that additional factors are important: perhaps this professor has also studied philosophy or is known to us to be a particularly trustworthy and astute individual whose opinion we have come to value?
In short, appealing to authority where the authority does know (or is expected to know) what he or she is talking about is a legitimate move in argument, but when the authority's expertise is not relevant then it is fallacious—indeed, a fallacy of relevance, as before.
Matters are not always so clear-cut, though. Even if the authority in question really is an authority in the field, it may be that the question under consideration is one of much controversy among his or her fellow academics. In our example, other philosophy professors may be found who say that philosophy is important, so that appealing to authorities on one or other side or an argument does no more than appraise us of what they think. Take another instance:
Professor Y, a highly respected biologist at a prestigious university, says that the likelihood of live evolving on Mars is so small that, for practical purposes, we can assume it didn't; therefore spending money on searching for life on the red planet is a waste of valuable resources.
Here the implicit idea behind the criticism is that with only a finite amount of money to go around and other deserving causes in need of support, why should we support a quest that academics like Professor Y agree is very likely to fail? Is this argument fallacious? It depends: we would need to know more information, such as whether the professor is an expert in the appropriate area of biology and if there is any controversy among similar experts. If the professor's opinion is indicative of the relevant biological community, then perhaps this is information we should keep in mind when forming an opinion on the issue? On the other hand, if the professor is something of a maverick and the weight of biological opinion goes against him or her, then appealing to him or her as an authority could be seen as fallacious, distracting us from the point at issue. In general, we need to be careful in assessing the value of expert testimony, as well as its relevance.
http://www.galilean-library.org/int16.html
http://www.nizkor.org/features/fallacies/appeal-to-authority.html