Drugs are chemicals that alter, block, or mimic chemical reactions in the brain. This causes an alteration of the body's normal processes, causing physical (Faster heartbeat, deeper respiration etc.), or mental (Elevated mood, new thought processes etc.) changes.
Drugs are used for a myriad of purposes, from anaesthesia, to psychotherapy, to just being able to wake up in the morning (Caffeine).
Drugs that are used for personal pleasure (often illegally) are called Recreational Drugs. Traditional RD's include marijuana, alcohol (Yes, it is a drug), MDMA (ecstasy), mushrooms, LSD, methamphetamine, heroin, crack/cocaine. Recreational drugs come under many criticisms, and praise by opponents, and users alike. Some believe the use of RD's to be immoral, and life-threatening, while others believe that new and thoughtful insights can come from drug use (especially psychedelics like mushrooms, and LSD)
Other drugs include caffeine, psychedelic mushrooms diphenhydramine, dimehydrinate, mesculine, morphine, dextromethorphan, PCP, and chocolate (Chocolate contains theobromine, which is an effective cough suppressant, possibly more effective than cough syrups, which are also drugs. Theobromine has mild mood elevating effects. It's also the reason you don't feed chocolate to your pets.)
Contrary to popular belief, drugs are not for stupid people. Stupid people are the reason that recreational substances cause such a problem in society.
If used responsibly, drugs can bring about new insights, and positive life changes, or can be used to just sit back and relax after a hard day's work. They can leave you with a better, and renewed appreciation of life. (This is usually the work of psychedelics) Keep in mind that responsible drug use is just that: responsible. Using certain drugs with high addiction potential, like cocaine, crack, heroin, and methamphetamine to name a few, is not responsible, unless you have amazing willpower.
Substances like marijuana, MDMA, and LSD to name a few can all be used responsibly, and can be very fun, but if used often, they can be detrimental to your health. LSD could warp your very perspectives on life to the point where you can't even look at certain things the same way anymore. Ecstasy is a neurotoxin which can leave you with long term depression. Finally, heavy marijuana use will damage your lungs (If smoked), and can leave you very unmotivated, and lethargic. It will also cause a mild cognitive impairment that will disappear after cessation of use.
In closing to this long rant, if you're intelligent, drugs won't make you stupid. If used responsibly, drugs can be a catalyst to a better, more motivated, and eventful life. As long as your drug use doesn't cross into other areas of your life (Work, school, social) you'll be alright.
On the other end of the spectrum, if you're an idiot and use drugs because it's hella fun, and cool, and you use them chronically, then they will probably catalyze your downfall. Especially if you blow guys at parties for your fix.
If you don't want to use drugs, that's fine, I respect that, and your friends should too. But please, all anti-drug people out there, don't bitch at people for doing them, and telling them that they are fucking up their lives. Unless they actually are. Blowing guys are parties, doing crack everyday, robbing convienience stores for drug money, and smoking pot in class are all signs that your friend is a fuck-up who needs to stop drugs.
Remember, it's not the drugs people use, it's the people who use them.