Depression

Depression is a serious illness. Health professionals use the terms 'depression', 'depressive illness' or 'clinical depression' to refer to something very different from the common experience of feeling miserable, or fed up, for a short period of time.

Doctors describe depression in the following three ways:

• by how serious it is - if it is mild, it is likely to have some impact on your daily life, if it is moderate, it is likely to have significant impact on your daily life and, if it is severe, daily activities may be almost impossible.
  
• by physical symptoms - if you have
  depression, you will probably have one or two (or maybe more) physical (somatic) symptoms, such as tiredness or a loss of appetite.
  
• by psychotic symptoms - if you have depression, you may also have psychotic symptoms, such as hallucinations, or delusions. However, these symptoms do not affect everyone with depression.
  

A few people still think that depression is 'not a real illness', and that it is some sort of 'weakness', or admission of failure. This is simply not true. Depression is a real illness with real effects, and it is certainly not a sign of failure. In fact, famous leaders, such as Winston Churchill, Abraham Lincoln and Mahatma Gandhi, all experienced bouts of depression.

More Info: (Incl. Links to Symptoms, Causes, Diagnosis & more)
http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=127