Wikipedia: Depression

Depression is a mood disorder characterised by loss of interest in life, anhedonia (inability to experience pleasure), lack of energy and motivation. Often it is accompanied by sleep disorders, especially early morning awakening and by feelings of greatly lowered self-worth. Depression is often described as being unable to feel -- even to feel many negative emotions such as sadness.

Psychiatrists have attempted to catagorize depression in many ways, but most agree that depressions fall into two broad catagories: Reactive or exogenous depression, which are depressions following an identifiable life trauma or loss; and endogenous depressions in which it is difficult to find an external cause. According to most psychiatrists, at least some endogenous depressions are related in a complex way to changes in some neurotransmitters in the brain. On one view, for example, changes in serotonin levels might both be caused by and be a cause of depression.

In addition, depression can be categorised into unipolar depression and depression as part of Bipolar Disorder.

In particular, Bipolar Disorder appears to have well-documented physical correlates, and responds well to chemical treatment.

It is theorised that unipolar depression and bipolar disorder may be part of a wider bipolar spectrum

Depressions are currently treated, with varying degrees of success, in several ways including: psychotherapy? (including [Freudian therapy]?, cognitive therapy, etc.), [anti-depressant medication]?, and [electroconvulsive treatment]?.