Depression is common; about 15 percent of people will have a bout of major depression at some point in their lives.
In depression you may have feelings of extreme sadness that can last for a long time. These feelings are severe enough to interfere with your daily life, and can last for weeks or months rather than days.
For more information
NHS Choices Symptoms, causes, diagnosis, treatment
BBC Health A guide to self-help and professional treatment
Depression Alliance Self-help groups