Addiction is defined as not having control over doing, taking or using something to the point where it could be harmful to you. The most well known addictions are gambling, drugs, alcohol and nicotine, but it's possible to be addicted to just about anything, including: your work, the internet, solvents and even shopping.
There are lots of reasons why addictions begin. In most cases it is due to they way these things affect the way you feel, both physically and mentally, being enjoyable to the point you have the urge to experience it again. This can develop into a habit that becomes very hard to stop.
Being addicted to something means that not having it causes withdrawal symptoms, or a "come down". Because this can be unpleasant, it's easier to carry on having or doing what you crave, and so the cycle continues.
Often, an addiction gets out of control because you need more and more to satisfy a craving and achieve the "high". All of this often has a seriously damaging affect on ones health work and social life.