How does a person know if he should seek treatment for depression since feeling moody is unavoidable? Life is not a bed of roses. All of us feel moody sometimes due to failures in life. This is perfectly normal. However, depression is abnormal.

How does a person know if he needs to seek treatment for depression, given that it is hard for him to distinguish whether the moody emotions are normal or abnormal? What are some symptoms to look out for?
TLDR

Many people say they are feeling depressed when really they are just suffering from a low mood.  Andrew Solomon said


  The opposite of depression is not happiness, but vitality. And it was vitality that seemed to seep away from me in that moment. Everything there was to do seemed like too much work. (Solomon, 2013)


This talk was about how depression affected him and his life, and is a good introduction to the subject.

The type of depression will depend on diagnostic criteria (see Different types of Depression below) but basically, depression is not just feeling sad.  When you are depressed, it is difficult to carry on with day-to-day life effectively for varying reasons, over a period of days or more.

Longer answer

There are many grey areas in the assessment of depression, just like there are with some other mental health problems.  The main traditional diagnostic systems used are currently the Diagnostic and Statistical Manual of Mental Disorders - DSM-5 (American Psychiatric Association, 2013) and the International Statistical Classification of Diseases and Related Health Problems - ICD-11 (WHO, 2018), and a formal diagnosis using the ICD-11 classification system is as follows:


  A [single] depressive episode is characterized by a period of almost daily depressed mood or diminished interest in activities lasting at least two weeks accompanied by other symptoms such as difficulty concentrating, feelings of worthlessness or excessive or inappropriate guilt, hopelessness, recurrent thoughts of death or suicide, changes in appetite or sleep, psychomotor agitation or retardation, and reduced energy or fatigue (ICD-11 6A70).
  Recurrent depressive disorder is characterized by a history or at least two depressive episodes separated by at least several months without significant mood disturbance (ICD-11 6A71).


The DSM-5 system requires match in at least five out of nine criteria for a diagnosis of major depression, although there are moves to improve the diagnostic procedures for mental disorders through systems such as HiTOP (Stony Brook Medicine, 2017; Kotov, et al., 2017).  However, one thing I think we can all agree on is that there are two different categories of depression.


Endogenous
Depression resulting from internal, possibly biological influences, as a result of possible genetic factors, or reactions to chemical/hormonal imbalances; and
Exogenous
Depression resulting from external influences in a person’s life, such as grief, loss or trauma.


Different types of Depression

There are several types of depression.


Unipolar Depression
The sufferer feels melancholic with no experience of any highs.  DSM-5 states that the criteria for diagnosis are the same as other forms of depression, however, the diagnosis is to be recorded with the specifier “with melancholic features” (p. 185).
Bipolar Disorder (BPD)
There are Bipolar I and Bipolar II disorders, both with severity levels of mild moderate and severe, and DSM-5 considers BPD to be a bridge between schizophrenia spectrum and other psychotic disorders in terms of symptoms, family history, and genetics; so, DSM-5 has a separated Bipolar Disorders from depressive disorders (p. 123).
Suicide risk for Bipolar Disorders is 15 times greater than the general population, and may account for 25% of all completed suicides (p. 131), with a prevalence rate of 36.3% of attempts for Bipolar I and 32.4% for Bipolar II and in terms of genetics, the risk of suicide seems to be 6½ times higher among first-degree relatives (a person's parent, sibling, or child) of those diagnosed with Bipolar II compared to Bipolar I    (p. 138)
Major Depression (Major Depressive Disorder in DSM-5 (p. 160))
Again, with severity levels of mild, moderate and severe, the module notes indicate that this is the most common form of depression.  Looking at DSM-5, diagnosis requires five (or more) of the diagnostic criteria to be present most of the day, nearly every day, during the same 2-week period.
Sufferers will feel down and uninterested in any activity external to them, and they are generally less motivated to deal with their basic needs.
Suicidal behaviour has a possibility of existing at all times and most completed suicides are not preceded by unsuccessful attempts, with an increased risk of completion within men, and those who are single or living alone (p. 167).
Mild Depression
This sounds to be a separate disorder, with the sufferer still able to engage in life but finding their activities harder to do.  However, looking at DSM-5, the criteria to look for are the same as Major Depressive Disorder, with few, if any, symptoms in excess of those required to make a diagnosis of Major Depressive Disorder (p. 162 &amp; 188). 
Atypical Depression
DSM-5 classifies this as the same as all other depressive disorders, but must have a specifier of “with atypical features”.  DSM-5 goes on to say that Atypical Depression, compared to the past, does not connote an uncommon or unusual clinical problem as the term might imply.  The levels of hypersomnia required are defined as at least 10 hours of sleep per day including daytime napping (or at least 2 hours more than when not depressed) (p. 186).
Persistent Depressive Disorder (previously Dysthymia)
Looking at DSM-5, this is a diagnosis which would be given when depression in any form is suffered for most of the day, for more days than not, over the last 2 years (p. 168), and the module notes help with other aspects by indicating that sufferers may not be aware of the condition and just see it as part of life, whilst feeling unimportant, dissatisfied and scared.
Psychotic Depression
DSM-5 states that the criteria is the same as other forms of depression, however, the diagnosis is to be recorded with the specifier


“with psychotic features”
(delusions and/hallucinations are present, of which some are mood-congruent and some are mood-incongruent),
“with mood-congruent psychotic features”
(the content of all delusions/hallucinations are consistent with the typical depressive themes of personal inadequacy, guilt, nihilism…) or
“with mood-incongruent psychotic features”
(the content of all delusions/hallucinations are not consistent with the typical depressive themes of personal inadequacy, guilt, nihilism…) (p. 185).

Seasonal Affective Disorder (SAD)
This is depression which affects the sufferer during certain seasons.  In most cases, SAD affects people during the autumn and winter months where daylight hours are less, but less commonly, there can be affects during the summer months due to heat and/or humidity or body image issues etc. (WebMD, 2017)  DSM-5 requires this to be recorded the same as any other depression, but with the specifier “with seasonal pattern” (p. 187). This enables you to not diminish the severity of the depression based on the fact that it is just seasonal.
Postnatal Depression
DSM-5 requires this to be recorded the same as any other depression, but with the specifier “with peripartum onset” (p. 186).  This enables you to not diminish the severity of the depression based on the fact that it is a result of giving birth to a child.
Premenstrual Dysphoric Disorder
There is a set of criteria laid out from page 171, with a requirement that the criteria must be met for most menstrual cycles that occurred in the preceding year and
in the majority of menstrual cycles, at least five symptoms must


be present in the final week before the onset of menses
start to improve within a few days after the onset of menses; and
become minimal or absent in the week post-menses



DSM-5 also talks about a depressive disorder which is specific to 7-18 year olds only, with onset before 10 years old, and that is Disruptive Mood Dysregulation Disorder (pp. 156-160).

References

American Psychiatric Association (2013). Diagnostic and Statistical Manual of Mental Disorders (5th Edition). Washington, DC: American Psychiatric Publishing.

Kotov, R. et al., 2017. The Hierarchical Taxonomy of Psychopathology (HiTOP): A dimensional alternative to traditional nosologies. Journal of Abnormal Psychology, 126(4), pp. 454-477. doi: 10.1037/abn0000258

Solomon, A. (2013). Depression, the secret we share — TED Talks. [Online]
Available at: https://www.ted.com/talks/andrew_solomon_depression_the_secret_we_share
[Accessed 8 June 2017].

Stony Brook Medicine (2017). The Hierarchical Taxonomy Of Psychopathology (HiTOP). [Online]
Available at: https://medicine.stonybrookmedicine.edu/HITOP/publications
[Accessed 23 May 2017].

WHO (2018). ICD-11. [Online] 
Available at: https://icd.who.int/browse11/l-m/en