There's one way to argue properly and about a thousand ways to argue improperly. Logical fallacies are so common, and tend to get so intricate, that even the most skilled skeptic can fall for a bad argument. So without further ado let's go on to our first fallacy.
The Argument from Popularity, also known as popular appeal or appeal to majority, has to be one of my favorite logical fallacies, simply because it is so simple, so intuitive (although wrong) that you can't go a few days or hours without running into it. In simple terms the argument from Popularity rest on the assumption that if a large number of people engage in some activity, then it must be the right thing to do.
The premise is that large numbers of people are not likely to be wrong. So if large numbers of people think A is right and B is wrong, then A must be right.
Of course the problem with this sort of argument lies with the premise. There is no reason to believe that large numbers of people who believe the same thing are right, simply by virtue of the large number. What happens if equally large numbers of people believe two contradictory things?
In general the fallacy goes like this:
Everybody likes/does/thinks/agree on A. Therefore A must be right.
However, we must be careful. IF the majority are experts in the field or subject that the argument applies to than it is not a fallacy to point out that all, or a majority of the experts agree on something. For example if we say "Most people floss, therefore flossing must be good for you" we are committing the ad populum fallacy (regardless if the conclusion is right or not. As long as the argument rest solely on the number of (non-expert) people that do something, it is a fallacy!) However, if we say "Most dentist recommend flossing, therefore flossing is good for you" we would not be committing a fallacy. Dentist are experts in dental care, therefore it is highly unlikely that a majority of the experts would be wrong (although not impossible). Even though it is possible for most experts to be wrong, it is not fallacious to present the fact that most of them agree on A, to support your claim that A is right.