What is the difference between legal and illegal drugs?

You have a asked a question that has a multi-faceted answer.

Generally, prescription 'drugs' are controlled substances. Their use, production, distribution and possession are all controlled by law. A prescription from a licensed doctor makes a controlled substance legal. The difference depends on such factors as how they were produced, distributed and in whose possession they exist. If you obtain powerful painkillers by a prescription written by your doctor. Your drugs are legal. If you buy those same painkillers on the street, they are illegal drugs. Also, any concoction made by unlicensed persons and sold on the street is an illegal drug although laws vary from state to state and country to country.