Depression is a medical illness that involves the mind and body. Also called major depression, major depressive disorder and clinical depression, it affects how you feel, think and behave. Depression can lead to a variety of emotional and physical problems. You may have trouble doing normal day-to-day activities, and depression may make you feel as if life isn't worth living.
More than just a bout of the blues, depression isn't a weakness, nor is it something that you can simply "snap out" of. Depression is a chronic illness that usually requires long-term treatment, like diabetes or high blood pressure. But don't get discouraged. Most people with depression feel better with medication, psychological counseling or other treatment.
To read more go to: http://www.mayoclinic.com/health/depression/DS00175/DSECTION=symptoms