What is depression -- psychologyinfo

Depression is a "whole-body" illness, involving your body, mood, and thoughts. It affects the way you eat and sleep, the way you feel about yourself, and the way you think about things. A depressive disorder is not the same as a passing blue mood. It is not a sign of personal weakness or a condition that can be willed or wished away. .... - What is depression
Signs of Clinical Depression: Symptoms to Watch For - webmd
Depression Symptoms, Warning Signs, Types, and Complications
'Depression Gene' Linked to Response to Stress - webmd