A depressive disorder is an illness that involves the body, mood, and thoughts. It interferes with daily life, normal functioning, and causes pain for both the person with the disorder and those who care about him or her. 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. People with a depressive illness cannot merely "pull themselves together" and get better. Without treatment, symptoms can last for weeks, months, or years.

Depression is a common but serious illness, and most people who experience it need treatment to get better. Appropriate treatment, however, can help most people who suffer from depression. Depressive disorders come in different forms, just as is the case with other illnesses such as heart disease. Three of the most common types of depressive disorders are described here. However, within these types there are variations in the number of symptoms as well as their severity and persistence.