Addiction means doing something but ending it is not in your hands. You are unable to stop when you want to, despite of the adverse consequences. It permeates your life, and your craving  increases with time. Addiction is about disrupting the processing of pleasure; the balance point is shifted so you keep creating more and more urges, and you keep wanting more and more.

KINDS OF ADDICTIONS
There are no fixed kinds as addiction can vary from person to person and sometimes it could be weird for others. But just naming few here:

1. Alcohol: About 18.7 million people are dependent on or abuse alcohol, and about 12,000 try it for the first time every day.
2. Drugs: An estimated 3.6 million people are dependent on drugs. More than half of first time users are female and younger than 18. Marijuana, cocaine and pain relievers are the leading drugs of abuse.
3. Tobacco
4. Caffeine: It’s the most widely used mood altering drug in the world and is routinely ingested by about 80% to 90% of Americans, primarily through soda and coffee.
5. Food: An addiction to food affects as many as 4 million U.S. adults and is strongly linked to depression.
6. Gambling.
7. Shopping: The addiction affects both genders almost equally. Cultural factors like advertising emphasis on the happiness products can bring, thought to fuel addictive buying.
8. Sex: An addict is dependent on the neurochemical changes that take place during sex and is consumed by sexual intercourse.
9. Internet: Interne addiction is thought to be an Impulse-control-disorder that can disrupt social relationships.

WHAT HAPPENS IN THE BRAIN DURING ADDICTION
We feel good when neurons in the reward pathway release a neurotransmitter called dopamine into the nucleus accumbens and other brain areas. Neurons in the reward pathway communicate by sending electrical signals down their axons. The signal is passed to the next neuron across a small gap called the synapse. Dopamine is released into the released into the synapse, crosses to the next neuron and binds to receptors, providing a jolt of pleasure. Excess dopamine is taken back up by the sending cell. Other nerve cells release GABA, an inhibitory neurotransmitter that works to prevent the receptor nerve from being over stimulated. Addictive substances increase the amount of dopamine in the synapse, heightening the feeling of pleasure, Addiction occurs when repeated drug use disrupts the normal balance of brain circuits that control rewards, memory and cognition, ultimately leading to compulsive drug taking.

Some people have a genetic predisposition to addiction. In the brains of addicts, there is reduced activity in the prefrontal cortex, where rational thought can override impulsive behavior. That can go for nonchemical addictions as well. Behaviors, from gambling to shopping to sex, may start out as habits but slide into addictions. Addiction is such a harmful behavior, in fact, that evolution should have long ago weeded it out of the population. Humans have always wanted to experiment with things to make them feel good.