A person who's addicted to cocaine has grown so used to the drug that he has to have it. Addiction can be physical, psychological, or both. Physical addiction is when a person's body becomes dependent on a drug. It also means that a person builds tolerance to a drug, and needs a larger dose to get the same effects. When a person who is physically addicted stops using drugs, he may experience withdrawal symptoms.
Withdrawal can be like having the flu - common symptoms are diarrhea and shaking.
Psychological addiction may happen along with physical addiction or on its own. In this case, the cravings for a drug are psychological, or mental. People who are psychologically addicted feel overcome by the desire to have a drug. They may lie or steal to get it. An addiction is not just measured by how many times a person uses a drug.