It is a drug, but there are different classes of drugs.

Caffiene is a drug, nicotine in tobacco is the drug, THC in the cannabis plant is a drug.

Some drugs are more addictive than others, but I find cannabis to be one of the only substances out there that people can start and stop whenever they please. When people think about it all the time, it's not that their body is going through severe withdrawls, it has just become a habit. It's something they are used to doing everyday, and then they stop, it's a hard habit to get out of.