This document defines depression as a serious medical condition that involves feelings of sadness, hopelessness, and lack of interest that interfere with daily life and last for weeks. It lists several forms of depression including major depressive disorder, dysthymic disorder, postpartum depression, bipolar disorder, and seasonal affective disorder. The document also discusses causes of depression, treatments including counseling, medication, and electroconvulsive therapy, comorbidities with other conditions, and ways to prevent depression such as exercise and stress reduction.