The following terms may also be used to describe specific types of depression:
Bipolar disorder (previously known as manic-depression) is a serious illness and those affected may experience depression lasting weeks or months, alternating with bouts of elation or mania ('highs') of variable duration. For people with elation, who do not have the accompanying depressive episodes, it is referred to as bipolar disorder.
Those affected by the illness may have 'normal' mood for months or even years. The mood-swings of bipolar disorder should not be confused with the mood changes that we all experience from time to time.
They are much more intense and prolonged and can have a devastating effect on the individual and their relationships. Have a look at the video below by Dr. Diana Cody which looks at Bipolar Affective Disorder in more detail.
These are old terms based on the supposed causes of the illness which are now rarely used. Reactive or exogenous depression, meaning coming from external factors, was seen as a depression brought on by life events or circumstances.
Endogenous meaning coming from within or having no apparent external cause, was believed to have a strong genetic basis i.e. it was hereditary. However now it is generally acknowledged that all cases of the depression are to some degree partly reactive and partly endogenous.
The severity of the illness is seen as more important than the cause and so as stated above depression is generally classified as mild, moderate or severe and treated accordingly.