Some people may assume that they are one and the same, but there are differences between the two states.
People can become dependent upon a substance, for example prescription drugs which often develops over a long period of time.
Their body has developed a tolerance to the drug and a larger dose is needed to achieve the same effect.
However, they are taking these drugs for a legitimate reason, e.g. pain relief rather than illegally. These are not usually taken for reasons of getting a ‘rush’ or ‘high’; rather they are taken to relieve a particular condition.
It may be the case that they don’t experience any physical or psychological effects when they stop taking these drugs.
But with an addiction, the person has developed a physical and psychological need for a substance and experiences cravings or withdrawal symptoms if they stop taking the drug. They take a drug or alcohol as a form of escapism or to deal with certain problems in their lives, e.g. stress.