Depression, also called clinical depression, dysthymic disorder, major depressive disorder, and unipolar depression is a serious medical illness that involves the brain. It's more than just a feeling of being "down in the dumps" or "blue" for a few days.
If you are one of the more than 20 million people in the United States who have depression, the feelings do not go away. They persist and interfere with your everyday life. Symptoms can include: sadness, loss of interest or pleasure in activities you used to enjoy, change in weight, difficulty sleeping or oversleeping, energy loss, feelings of worthlessness, thoughts of death or suicide.
Depression can run in families, and usually starts between the ages of 15 and 30. It is much more common in women.
National Institute of Mental Health: http://www.nlm.nih.gov/medlineplus/depression.html