Problem gambling is an urge to gamble despite harmful negative consequences or a desire to stop. The term is preferred to compulsive gambling among many professionals, as few people described by the term experience true compulsions in the clinical sense of the word. Problem gambling often is defined by whether harm is experienced by the gambler or others, rather than by the gambler's behavior. Severe problem gambling may be diagnosed as clinical pathological gambling if the gambler meets certain criteria.
Extreme cases of problem gambling may cross over into the realm of mental disorders. Pathological gambling was recognized as a psychiatric disorder in the DSM-III, but the criteria were significantly reworked based on large-scale studies and statistical methods for the DSM-IV. As defined by American Psychiatric Association, pathological gambling is an impulse control disorder that is a chronic and progressive mental illness.
Pathological gambling is now defined as persistent and recurrent maladaptive gambling behavior meeting at least five of the following criteria, as long as these behaviors are not better explained by a manic episode:
Preoccupation. The subject has frequent thoughts about gambling experiences, whether past, future, or fantasy.
Tolerance. As with drug tolerance, the subject requires larger or more frequent wagers to experience the same "rush".
Withdrawal. Restlessness or irritability associated with attempts to cease or reduce gambling.
Escape. The subject gambles to improve mood or escape problems.
Chasing. The subject tries to win back gambling losses with more gambling.
Lying. The subject tries to hide the extent of his or her gambling by lying to family, friends, or therapists.
Loss of control. The person has unsuccessfully attempted to reduce gambling.
Illegal acts. The person has broken the law in order to obtain gambling money or recover gambling losses. This may include acts of theft, embezzlement, fraud, forgery, or bad checks.
Risked significant relationship. The person gambles despite risking or losing a relationship, job, or other significant opportunity.
Bailout. The person turns to family, friends, or another third party for financial assistance as a result of gambling.
Biological Bases. The person has a lack of norepinephrine.
As with many disorders, the DSM-IV definition of pathological gambling is widely accepted and used as a basis for research and clinical practice internationally.

Info Source, Links and more info:
http://en.wikipedia.org/wiki/Pathological_gambling