These are three different effects caused by certain psychotropic drugs.
Hallucinogens cause hallucinations, which is an effect on the brain that causes the person experiencing he hallucination to experience things that are not really happening. This is typically thought of as seeing things that are not there, but hallucination can also be any of the five senses. Most typical hallucinations are experienced visually and aurally. Which is to say, they are seen and heard.
Stimulants cause the person to be more “up,” like the feeling one has when they’re excited about something. They have the effect of causing a person to be more alert.
Depressants cause a person to be “down,” like the feeling one has when they are tired.
Keep in mind that all three of these effects are not mutually exclusive, and are often side effects of drugs designed to address other things, although some drugs are made to specifically cause these effects. For example:
LSD, which is also known as Acid, is a typically recreational drug – though it has been used in certain laboratory studies – which was designed just for its hallucinogenic effects. Some people take it for fun, but that is seriously not recommended. It can have long term mental effects that you do not want.
Certain ADD/ADHD medications are stimulants. These help people with ADD/ADHD to focus, and these drugs, like Adderall are designed to cause that effect.
Certain drugs, like Valium, are designed to cause the person to experience the depressant effects. This is not to be confused with the person being depressed in terms of their emotional state, rather that the person will feel relaxed. These drugs can be helpful to a person who has experienced some trauma, or is experiencing severe anxiety.
Again, no drug should be taken without a doctor’s supervision
Also, I am not a doctor, I have just learned these things by being 43 years old.