Depression is a state of low mood that is described differently by people who experience it. Commonly described are feelings of sadness, despair, emptiness, or loss of interest or pleasure in nearly all things. Depression can also be experienced in other disorders, such as bipolar disorder (manic-depressive disorder).

Schizophrenia is a mental illness in which the person suffers from distorted thinking, hallucinations, and a reduced ability to feel normal emotions.

Anxiety is a complex combination of the feeling of fear, apprehension and worry often accompanied by physical sensations such as palpitations, chest pain and/or shortness of breath. It may exist as a primary brain disorder or may be associated with other medical problems including other psychiatric disorders.

Bipolar disorder (previously known as Manic Depression) is a psychiatric diagnostic category describing a class of mood disorders in which the person experiences clinical depression and/or mania, hypomania, and/or mixed states.