Addiction, a disease.
Addiction has a physical and a psychological aspect. Right away physical addiction we mean that our brains adapt to the use of the substance, so you always need more of it to get the same effect. This mechanism is actually a kind of protective mechanism for the brain. It allows an alcoholic, for example, to move amounts of drink that could be fatal for someone who drinks little or never. If you stop using the drug, you sometimes get unpleasant withdrawal symptoms. That's why quitting is often so difficult. But not every substance is physically (equally) addictive.
De psychological addiction means that you crave the effect that the drug causes you. You have learned that a glass of wine, a cigarette, or a shot of heroin can do you good, making the temptation particularly strong even if you have physically taken off the drug in question.