Argumentum ad populum is Latin for 'appeal to the people'. Arguments who commit this logical fallacy are arguments which support a particular proposition on the grounds that the majority, or a large group, of people believe that proposition.
The form of this fallacious type of argument says that A is true simply because the majority of people believe that A is true, supposing that the majority cannot be wrong.
- Thousands of martyrs died defending Christianity, therefore Christianity must be the right religion. So many people could not have been wrong.
- 60% of Americans do not believe in the theory of evolution, therefore it cannot be right.
- Everyone else in this meeting adopted my position, so you should do so as well.
- There was no opposition to this new law so I guess it must be fair and exactly what people wanted.
The reason this is considered a fallacy lies on the case that the amount of people believing a proposition does not indicate whether that proposition is true or false. People's belief system does not have any effect on the proposition itself and, in fact, the proposition would be true even if there were no minds to perceive it.
The case that 75% of Americans believe that the earth orbits the sun  cannot be an indicator that that particular proposition is true, but it must be weighed upon scientific theories and mathematical calculations, based upon existing models of the solar system, to assess the proposition.
When it comes to more advanced theories or findings, however, things become a bit more complicated. On the above grounds, one might argue that the fact that 97% of scientists believe in human-caused global warming means that human-caused global warming is true.  (Linked with the next fallacy: Appeal to authority). That's a mistaken way to approach the issue. The fact that 97% of scientists believe in human-caused global warming only raises the possibility that that proposition is true. It in no way guarantees it. What guarantees the truth of the proposition is the experiments that have been done, and the theories that were proposed to test the proposition, published in peer-reviewed journals. This is the reason, therefore, why appealing to the majority is always a fallacious argument and must be in any case avoided.