A folk remedy is any purported health remedy which comes from a pre-scientific source, usually through oral tradition. They remain popular among people who continue to use them because it is popular in the culture they are from or because "this is what grandma did", but many such practices have also remained in use through promotion as patent medicines or alternative medicine fads.
Folk remedies are distinguished from much alternative medicine and mainstream, scientifically-proven medicine alike by predating the era of modern medicine in origin. They range from sometimes effective to not at all effective. Some folk remedies may be potentially provable, others clearly not. It should be noted that the fact that your great-grandparents swore by such remedies doesn't give them some mystical superiority to modern medicine. The opposite is quite often the case, as these remedies came about by people making do with what was on hand in the era before medicines proven by the scientific method became readily available. Common ingredients in folk remedies in the United States include things like turpentine, sassafras tea, and whiskey. Some folk remedies also exist that are more in the realm of magic than medicine at all.
"Traditional medicine" systems also of pre-scientific origin, such as traditional Chinese medicine (including acupuncture), indigenous tribal medicine (including shamanic practices), and homeopathy, are a related practice with the main distinction being that folk remedies are a mish-mash of practices not part of any system per se.
It could be said that folk remedies, far from being a thing of the past, are still very much part of modern culture because we still have an oral tradition and equivalent means of passing along purported folk remedies, including over the internet. Thus somebody who tries colonics or some exotic fad diet might gush with enthusiasm to their friends, who will try them solely on the basis of word of mouth without doing further research to determine their actual effectiveness. Modern folk remedies include such things as tea tree oil, colloidal silver, shiitake mushrooms, DMSO, and ear candling.
- Warts - numerous folk remedies exist for warts. Because warts often disappear on their own, somebody might notice that a wart went away after rubbing a penny on it or putting turpentine on it, and pass this "remedy" down through oral tradition. An important concept to remember here is correlation does not equal causation.
- Faith healing
- Whiskey - as an all-purpose pain reliever and antiseptic, purported snake bite remedy, and numerous other uses.
- Snake oil, and exotic animal parts such as ground-up horns, are old folk remedies popular in China. Their continued use has created a problem with poaching and international smuggling of animal parts of endangered species.
- Prune juice - as a natural laxative.
- Burns - some folk remedies claimed to use quasi-magical practices to "draw out the fire" from the burn, a mystical pre-scientific notion. Others involved putting such things as lard, grease, turpentine, or butter on the burn. As any paramedic today will tell you, putting these on a burn does more harm than good.
- Chewing tobacco - to cure athlete's foot and worms.
- Using dock leaves to relieve the rash caused by stinging nettles.
 Bottom line
There is probably no harm in trying benign folk remedies for benign conditions, but if you have a serious medical condition or a medical emergency such as poisonous snake bite, don't rely on great-grandpa's old cure for snake bite; instead, see a physician. There is a good reason why life expectancy back then was less than half of what it is today.
 See also