Most people have changes in mood from time to time, sometimes feeling happy and sometimes feeling down.  The change in mood that signifies depression is of a different degree, a lowering of mood that is oppressive and tends to effect most aspects of life.

Depression can vary in severity from mild to very severe.  The most prominent symptom is a low mood that may be generally 'feeling down' for much of the time in mild depression, to feeling unremitting, crushing despair and misery in severe depression.  Other symptoms of depression that are common, and also present in varying degrees of severity, include loss of concentration, loss of appetite, and difficulty sleeping. 

Sometimes there is a clear and understandable reason why someone may be depressed, for example, experiencing loss of some kind is a common precursor to low mood.  At other times though, the depression may seem 'to come out of the blue' with no obvious cause. 

Antidepressant medication is commonly prescribed for depression by GPs and psychiatrists, and indeed is particularly indicated for people with more severe symptoms and for those for whom the depression seems to 'come out of the blue'.  However, many people have an inkling of why they are depressed and prefer to deal with it by using psychological therapy of one type or another, or by combining it with medication.  Choosing a psychological therapy may be more painful in the short term as it often means facing up to difficult and painful things, but if successful it is also more likely to last as a treatment and to make recurrence of the depression less likely.