Depression in general is characterized by a melancholic mood, a loss of interest, appetite and energy, problems with concentrating and restlessness, and even cognitive or motor difficulties. In severe cases, even delusions and hallucinations may be present. Of course, every individual experiences a unique combination of these symptoms.

Length of Condition

Depression is a general term used to denote several similar conditions. A division based on the time the depression lasts yields two distinct types:

Acute depression: this type of depression is characterized by a severe manifestation of the aforementioned symptoms, and lasts at least two weeks.
Chronic depression: this type of depression, also called dysthymia, leads to a less severe expression of the symptoms, but typically lasts a lot longer, at least two years.

Presence of Other Problems

Another division that is used to classify types of depression, is based upon the presence or absence of otherpsychological or physical problems. Here, also two types can be discerned:

Primary depression: the occurrence of depression (or major depressive disorder) without a correlation with other psychological or physical problems (both acute and chronic depression are an example of this type of depression).
Secondary depression: this type of depression results from other psychological or physical problems. For example, someone grieving for a friend or loved one can exhibit symptoms of depression as response to this emotional stimulus. Usually, this type is rather short-lived and these individuals can mostly recover on their own.

This division, however, might pose some problems, as it can be argued that all types of depression have a physiological root (hormone concentrations, lack of certain nutrients, such as omega 3, and even genetic differences)

Steady or Swinging Mood

A third type of division is based on the presence or absence of periods of elevated mood. Depending on the steady or swinging nature of the mood, two types of depression can be discerned:

Bipolar depression: this type is characterized by at least one period of mania, often involving extreme feelings of confidence, power and creative energy.
Unipolar depression: this type of depression lacks the presence of periods of elevated mood, and the person suffers from a constant lack of self-esteem, energy, interest, … .

Conclusion

So, depressed individuals differ in the length of their condition, the presence of other psychological or physical dysfunctions and the steady or swinging nature of their mood. These divisions might be important as they might reflect deeper differences on the level of hormones or genes. This also means that these different types of depression might require different types of treatment.

References

Breedlove, S.M., Rosenzweig, M.R. & Watson, N.V. (2005). Biological Psychology: An introduction to behavioral and cognitive neuroscience. Sinauer Associates Inc.
Nelson, R. (2005). An introduction to behavioral endocrinology. Sinauer Associates Inc.