Addiction is when the body or mind badly wants or needs something in order to work right. When you have addiction to something it is called being addicted or being an addict. People can be addicted to drugs, cigarettes, alcohol, gambling, caffeine, and many other things.
When somebody is addicted to something, they can become sick if they do not get the thing they are addicted to. But taking more of the thing they are addicted to can also hurt their health. Some people who are addicts need to go to a doctor or hospital to cure the addiction, so they no longer crave (want or need) the drug.
Addiction takes place when the addictive element enters the body. The blood stream moves the element around the body so that it gets to the brain, and makes the person feel that they are enjoying it; then they want more of that substance. This takes place remarkably fast for some addictions such as hashish and cocaine; but all of them control the consumer. Once under this control, the consumer does not have a completely free will, and listens to the addiction's bidding.
People can also be addicted to other things which are not drugs. You may hear people talk about being "addicted to the Internet" or "addicted to chocolate". This means that they get into a habit of enjoying that thing, so if they have to go without it for a while, they miss it a lot.