Abnormal psychology studies the nature of psychopathology, its causes, and its treatments. Of course the definition of what constitutes 'abnormal' has varied across time and across cultures. Individuals also vary in what they regard as normal or abnormal behaviour.
In North America the 'bible' of abnormal psychology and psychiatry is the Diagnostic and Statistical Manual of the American Psychiatric Association.The current version of the book is known as DSM IV-TR. It lists a set of disorders and provides detailed descriptions on what constitutes a disorder such as Major Depression or Anxiety Disorder. It also gives general descriptions of how frequent the disorder occurs in the general population, whether it is more common in males or females and other such facts. The diagnostic process uses five dimensions called 'axes' to ascertain symptoms and overall functioning of the individual.