1) Tobacco: Extremely dangerous if abused, which it always is, except in some Native American cultures long ago. People smoke tobacco because they start in their teens when "everyone is doing it," and want to be cool. However, due to nicotine, they get hooked.
2) Nicotine: Possibly extremely helpful chemical, attaches to receptors in our brains and improves memory, concentration, and is possibly neuroprotective, helping to prevent such things as Alzheimer's. "Possibly" is a key word. EXTREMELY ADDICTIVE. Many new antidepressants and anti anxiety agents are currently in trials using nicotinic receptor agonists.
3) Marijuana: A naturally growing flower of the hemp plant. There are two strains, Sativa and Indica. Both contain two main ingredients, THC and CBD. Sativa is supposed to make you more awake and "high" and Indica tends to make one sleepy or "stoned."
THC is the one that makes people get "high" or "stoned." Impairs a person's ability to retain new information and recall it for about 24 hours from the time it is smoked. Therefore, if one smokes every day, one may have some difficulty with complex tasks. However, it does seem to improve creativity and can help some feel relaxed, less anxious.
On the other hand, there do seem to be a lot of people who smoke marijuana and become quite paranoid, experience time dilation or slowing, and feel anxious or uncomfortable in social situations. Chronic marijuana abuse does also seem to lead to depression and anxiety, although it differs depending on the strain and the person.
Marijuana of today, particularly in states where it is legal or medically legal, is extremely potent, with high levels of THC in most strains. The marijuana of the 60's was not like the marijuana of today.
CBD, the other chemical, seems to have neuroprotective effects, and there is some research which shows that it theoretically should help prevent Parkinson's. CBD is not psychoactive in that it does not make you "high," but rather relaxes you. It's almost like a natural Xanax, in my opinion, albeit weaker.
Due to the lack of long-term research of marijuana, it is difficult to determine all the health effects, but it has been used for centuries in a variety of ways. Hemp is a particularly sustainable part of the plant which can be used instead of trees for paper, rope, etc.
As far as its effects on the lungs, it is not clear that it causes emphysema (like tobacco does) or cancer (like tobacco does).
One problem with marijuana is that you are basically playing chemistry set with your brain. Despite the derth of research and anecdotal evidence that marijuana can be helpful, many chronic users can feel depressed, anti-social, and anxious. If one is younger than 25 or so, the best thing to do would be to allow the brain to mature before starting to smoke or drink or do anything.
Unfortunately, college or even high school is where most people smoke their first joint.
4) Alcohol: Deadly poison. Really doesn't have many good effects. Causes relaxation and improved mood at first, then tends to increases aggressiveness, make one extremely impulsive, then causes blackouts in some and hangovers for most. As one abuses alcohol, the hangovers increase in frequency and severity, causing severe depression, anxiety, etc. In severe cases, one might get the DT's, or delirium tremens, which causes psychosis and sometimes death. One can also get permanent brain damage or an encephalopathy from alcohol called Wernicke-Korsakoff syndrome.
It is interesting to note the stigma against marijuana when alcohol is almost praised in the US, advertised heavily at all major sporting events, etc, and alcohol is certainly worse than marijuana as far as people committing crimes, engaging in violence, and generally getting brain damage.
