What are drugs?
"Drugs are defined by U.S. law as any substance (other than a food or device) intended for use in the diagnosis, cure, relief, treatment, or prevention of disease or intended to affect the structure or function of the body. (e.g.) Oral contraceptives are an example of drugs that affect the function of the body rather than a disease.” In simple words a drug is any chemical substance that affects the body and its processes. Some people incorrectly use the word drug to mean only a substance that produces a pleasurable sensation. Some drugs with potential for abuse have legitimate medical purposes, and others do not.
Also, a drug is a substance that when put into the body can change the way the body works.
A "soft" drug is less addictive and considered less harmful to the body and to society as a whole.
A "hard" drug is harshly addictive and considered much more harmful to the body and society in general.
Psychoactive drugs
Definition- These are drugs that change your feelings, your perceptions and your behaviour. Psychoactive drugs bring about a chemical reaction in the brain affecting feelings, thoughts and behaviour.
The above quoted sentence belongs to the website
(http://www.merckmanuals.com/home/drugs/overview_of_drugs/overview_of_drugs.html)