For every atom belonging to me as good belongs to you. - Walt Whitman
Making a formal psychological diagnosis can be a mixed blessing. It has several potential advantages. If many of my symptoms can be accurately grouped under a specific psychological diagnosis, it may well help to understand what is happening, to clarify the likely time course of my symptoms, and to choose treatments that have the best chance of being effective. It's worth noting that often people suffer from more than one psychological disorder at the same time - this is called comorbidity and it is common.
Sometimes putting different individuals into the same diagnostic group can obscure the fact that we are all unique in so many important ways - our life histories, hopes, values, living situations, biochemistries, fears, understandings, and so on. Diagnosis is often helpful but should be seen as only part of a bigger picture. In this psychological section of "Good Knowledge", diagnostic criteria are taken from the American Psychiatric Association's Diagnostic and Statistical Manuals of Mental Disorders (DSM) version IV, published in 1994. The other major classification system which includes psychological disorders is the International Classification of Diseases (ICD) published by the World Health Organization. The ICD psychological diagnostic categories overlap very considerably with those of DSM-IV.
There are descriptions and diagnostic criteria for over 50 of the commonest mental disorders at Canadian psychiatrist Dr Phillip Long's fine website - Internet Mental Health. This site also contains a wealth of other useful information about mental disorders. It's well worth a visit.