The first logical fallacy I'm going to cover is Argumentum ad populum. That translates to appeal to the people if you don't want to remember the fancy Latin.
This is the fallacy your mom is warning you about when she asks you 'if all your friends jumped off a cliff, would you jump too?' In that case, all of your friends are idiots that falsely believe jumping won't leave them a slimy smear at the bottom of the cliff, and because you think the majority couldn't possibly be wrong, you'll soon be a slimy smear too.
So basically, if the majority believes something, that doesn't mean the majority is right. Like a lot of fallacies, this gets used in advertising all the time—Four out of five dentists agree; use the #1 cleaning detergent in the country; buy our boner pills, so many people have tried them, they must work, right? From those arguments, your can infer a truth but not prove it. In other words, they're inductive rather than deductive arguments.
If you'd like to know the truth about something you're better off searching for the verifiable facts and deciding for yourself. In your mother's example, you can look over the edge of the cliff, see that all of your friends have died a horrible gruesome death, and decide that maybe getting chummy with a suicide cult probably wasn't the best of ideas.
There are some situations where this fallacy is NOT a fallacy. The biggest is when it's used to describe widely accepted rules of safety. I'm not talking 'don't walk under a ladder' here. Think more along the lines of traffic rules. Most people in the US agree that driving on the right side of the road is the right thing to do, so that is the right thing to do in the US if you don't want to be squashed like a Pinto at a monster truck rally on Sunday, Sunday, Sunday.
You could also say that democracy and social convention do not apply. The majority of the club has agreed to vote out Steve because he's an asshole, therefore Steve is voted out. If the conclusion was 'therefore Steve is an asshole' that would still be a fallacy. Most people on the planet Flatula think it's polite to fart by way of greeting, therefore, on the planet Flatula, it probably wouldn't go amiss to have a bit of a toot before you introduce yourself. Of course, rules of social convention change all the time, so your mileage may vary.
Some people might argue that morals are also immune to this fallacy, but 4 out of 5 philosophers agree that that's absolute shit.