What is depression?

Let's check a simple example. You get a bad mark. You are not satisfied with your studying, you feel sad and upset about this event, but you see the way out. It is natural and normal. It means that you able to cope with your anxiety and go on your usual lifestyle.
Depression is to be specified in comparison with simple sadness. Depression makes you feel despair, hopelessness, weakness, ignorance, boring only. It seems to you that nobody can help you. You stay at home away of your friends and relatives for a day, a week or even months. You do not want to lead your usual lifestyle; you do not feel any strength and desire to do something. Your single desire is to stay alone, away of the world with its troubles or your own problems which are left in the outer world. You can not eat and sleep properly, your body aches without real reasons. Sometimes you feel splashes of excitement or joy, but it disappears very quickly, being replaced by harder sadness and despair.
Usually this condition lasts for two weeks regularly. In this case the state is to be marked as depression.
Stress or too intensive and long worrying (for example, about somebody's death) could cause your depression. Sometimes the reasons are genetic, but rarely genetic diathesis develops into depression. For that someone needs specific social events and conditions.