Depression is one of the mental illness. It should not be consider similar to sadness of the mood. Sadness of mood is one of the normal human emotion which would not last longer ( just like other emotions like being in happy mood for some time). But depression is more than just sad mood as it involves your many areas of the life like difficulties at work and professional life , social life etc.
Depression is categorized according to its severity and it's treatment varies accordingly.
To identify the depression 3 most important areas to remember are-
1) Low mood- eg. You feel low throughout the day and night
2) Lack of interest/ Anhedonia- eg. You start loosing interest in many areas of life which you used to enjoy earlier. (like not getting pleasure in your hobbies- reading, gaming, chatting with your friends etc)
3) Lack of energy/ easy fatigability- eg.you feel fatigued even on routine activities which was not the case earlier.
These are 3 important areas which gets affected.
Other additional symptoms include
Loss of appetite,
Feeling helpless, hopeless, unworthiness
Pessimistic views of future,
Reduced self confidence
If anyone having above mentioned symptoms (for 2 weeks or more ) should seek help of mental health professionals ( Psychiatrist, Psychologists etc)
Treatment varies depending on severity which includes medications , therapy , or combination of both.