Technically a drug is anything you put into your body that is not required as food. This makes it a very broad term, as I would argue that sugar is a drug- not required as food..... regardless, drugs can do just about anything, and they can do nothing. There are drugs to wake you up, and put you to sleep. Stop pain and maintain erections. Drugs can make you hallucinate and they can hit the pleasure center of your brain to make you want more. They can make you sick and they can kill. They can save lives.
Drugs are another word for "medicine." Prescription drugs are suggested by doctors to help you feel better. However, if you get addicted to them/take more than what you're supposed to, it can be detrimental. There are also non-prescription drugs that people make that are also very dangerous. Some people "snort" them, and other people inject them with a shot. To make a long story short, if they're not from the doc, don't try them!