Psychopathology is a term which refers to either the study of mental illness or mental distress or the manifestation of behaviours and experiences which may be indicative of mental illness or psychological impairment.
The many different professions may be involved in studying mental illness or distress.
Most notably, psychiatrists and clinical psychologists are particularly interested in this area and may either be involved in clinical treatment of mental illness, or research into the origin, development and manifestations of such states, or often, both.
More widely, many different specialties may be involved in the study of psychopathology.
For example, a neuroscientist may focus on brain changes related to mental illness.
Therefore, someone who is referred to as a psychopathologist, may be one of any number of professions who have specialised in studying this area.
Psychiatrists in particular are interested in descriptive psychopathology, which has the aim of describing the symptoms and syndromes of mental illness.
This is both for the diagnosis of individual patients (to see whether the patient's experience fits any pre-existing classification), or for the creation of diagnostic systems (such as the Diagnostic and Statistical Manual of Mental Disorders) which define exactly which signs and symptoms should make up a diagnosis, and how experiences and behaviours should be grouped in particular diagnoses (e.g. clinical depression, schizophrenia).
Psychopathology is not the same as psychopathy, which has to do with antisocial personality disorders and criminality.