A comprehensive explanation of the illness of depression is described by the National Institues of Mental Health (NIMH) in a brochure called Plain Talk About Depression.

There is a difference between just feeling blue, and the illness of depression.

The brochure explains:
[list]What is a Depressive Disorder?
Types of Depression
Symptoms of Depression and Mania
Causes of Depression
Diagnostic Evaluation and Treatment
Psychotherapies
How to Help Yourself If You Are Depressed
Where to Get Help[/list:u]
A copy of Plain Talk About Depression suitable for printing can be downloaded Click Here