Rats, tattletails, squeelers, narcs, or basically people who don't know how to mind their own business and report someone to the police, boss at work, or other authority figure for something that doesn't concern them.
Happens all the time. Someone brags about smoking weed, for example, and some asshole decides to report them. (No, this isn't something that ever happened to me or mine, but it does happen.) Or maybe you stumble onto a facebook page, gossip about it to the wrong person, and get someone fired...
My creedo: See no evil, hear no evil, speak no evil. Never name names, never mess with someone that didn't mess with me first.. That doesn't mean I wouldn't cooperate with the law over a serious crime, but a lot of the stuff people get turned in for is just petty stuff, and I prefer minding my own business if it isn't over something serious..
Wish more people thought this way.