Smoking is when you ingest some sort of burnt substance such as tobacco, Marijuana, Methamphetamine, Crack cocaine, Black tar heroin and many other including prescription drugs. for many years smoking has been a "activity" to get some long or short term effect. Smoking can either be lethal or helpful depending on the substance. One example of a drug that's been used a lot over the years would be marijuana. Marijuana can affect other's with a euphoric high. That high is properly called THC (Tetrahydrocannabinol)
As much as some substances can help someone, some smoking substances can be very harmful or even cause DEATH. Some examples of dangerous substances would be Black Tar heroin, Cigarites, and cocaine