Addiction
There is a psychological/physical component; the person is unable to control the aspects of the addiction without help because of the mental or physical conditions involved.

Habit
It is done by choice. The person with the habit can choose to stop, and will subsequently stop successfully if they want to. The psychological/physical component is not an issue as it is with an addiction.

Put simply
With a habit you are in control of your choices, with an addiction you are not in control of your choices.

Addiction to substances or activities can sometimes lead to serious problems at home, work, school and socially.

The causes of addiction vary considerably, and are not often fully understood. They are generally caused by a combination of physical, mental, circumstantial and emotional factors.

Addiction, often referred to as dependency often leads to tolerance - the addicted person needs larger and more regular amounts of whatever they are addicted to in order to receive the same effect. Often, the initial reward is no longer felt, and the addiction continues because withdrawal is so unpleasant.

According to Med lexicon’s Medical Dictionary:
Addiction is Habitual psychological or physiologic dependence on a substance or practice that is beyond voluntary control. Withdrawal has many meanings, one of which is A psychological and/or physical syndrome caused by the abrupt cessation of the use of a drug in an habituated person.

According to the current Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), published by the American Psychiatric Association

Substance dependence is When an individual persists in use of alcohol or other drugs despite problems related to use of the substance, substance dependence may be diagnosed. Compulsive and repetitive use may result in tolerance to the effect of the drug and withdrawal symptoms when use is reduced or stopped. This, along with Substance Abuse are considered Substance Use Disorders..

Addiction may include prescription medications
Most people think of illegal drugs when they hear the word “addiction”. However, prescription drug addiction is becoming a serious public health problem in the USA and many other nations. Prescription medication abuse was described as “an epidemic” by researchers from the University of Nebraska Medical Center College of Medicine in a study they reported on in November 2012.

The scientists explained that doctors today treat pain differently than they used to years ago. This change has led to an increase in prescription drug abuse.

In the USA in the 1990s - the decade of pain treatment - not only was there a change in medication, but also policy. Pain became the fifth vital sign doctors looked out for, along with respiratory rate, blood pressure, body temperature and pulse rate. Doctors’ offices today commonly have signs in their waiting rooms asking patients to rate their experience of physical pain from 1 to 10.