The Diagnostic and Statistical Manual of Mental Disorders (DSM) is a catalog of mental disorders used by doctors (including psychiatrists), psychologists, and other mental health workers, as well as insurance companies, to classify individuals with diagnosable mental disorders.
In the past the DSM would be updated every few years and the letters following DSM noted which update was being referred to. The most recent DSM is the DSM-5, which was released in 2013. With this update the title changed from having a Roman number (IV) to the number 5, and the intention is to allow the document to be updated in an ongoing manner, rather than having one big update every few years.
Documents like the DSM are often presented as medical fact by those involved in their development. Critics argue that the diagnoses are impossible to separate from politics, culture, and society, and they are in fact much more contextual and political than objective. Critics, including mental health providers and researchers as well as those who are in the mental health system and receive diagnoses, point out that the criteria for what makes someone diagnosable have as much to do with things like one's race, class, access to education, overall health status, than they do with something organic in the body or the mind.
One potential benefit of a text like the DSM is that it gives people working in the area of mental health a common language (albeit a language that is clinical and sometimes difficult to follow).
There are many potential problems with the DSM, not least of which is that it can be considered to be the “final word” or offering objective facts regarding mental health. In reality the content of the DSM is subject to cultural and historical influences, and what is now considered a mental illness may not be considered one at a later date.
A good recent example of this is homosexuality, which was considered a mental illness until it was removed from the DSM in 1973. The difference between being gay in 1972 (when you would have been called mentally ill) and 1974 (when you wouldn't have, at least not by a psychiatrist) has nothing to do with an individual and their orientation and everything to do with a medical system that denies how influenced it is by social and political variables.