Logical fallacies are types of arguments that are invalid. Once you know the logical fallacies, you can recognize them in arguments constantly. Knowing these can really help to decipher what is true and is what is pure baloney.
The first logical fallacy I wanted to do is a clear example of just how ridiculous these arguments can be. The argument basically says lots of people agree with what I am saying, therefore I am right. The number of people who agree with something has no bearing on whether or not it is true. For thousands of years people believed the Earth was flat; that didn't make it true.
You hear this fallacy used in advertisements a lot. They try to convince you that because other people use their product, it works. Just think of how many times you have heard the line "the trend that is sweeping the nation." This says nothing about the product being advertised except that it is widely used. Don't believe in or use something just because it's popular. Look for the evidence.