Depression is one type of Mood Disorders (others are Dysthymia and Bipolar Disorders). Severe depression or Major Depressive Disorder (scientific name) is characterized by a intense sadness and lack of pleasure in activities that were previously enjoyable (e.g. movies, hobbies), changes in sleep or eating patterns, feelings of worthlessness , hopelessness or guilt, concentration and memory problems, indecisiveness, slowed activity, fatigue, ruminations over perceived past mistakes, and sometimes suicidal ideation, plan or attempt. Some people present somatic complaints rather than feelings of sadness and many may deny feeling depressed. Others may experience constant feelings of irritability and act in a hostile or accusatory way towards others.
The most effective treatment for Major Depressive Disorder is Cognitive Behavioral Therapy as well as a relatively new clinical modality named Behavioral Activation developed by Drs. Martell and Addis. Depressive patients who participate in these therapies are likely to experience a marked improvement in their mood in a relatively short time.