Depression is mainly a mood disorder, which can have a vast variety of symptoms, most of which are usually not related to depression by a person who experiences it. Depression is not simply sad or disappointed about something in your life. Depression is something that changes everything you life; the way you think, the way you feel about your daily routine, how you eat, sleep and even comprehend. It seems to be colored all in black or gray; you might feel helpless, hopeless, worthless, without any relief, etc.
Depression and its form vary from person to person. Still, psychiatrists define several common symptoms, which are considered to be signs of depression. Also, there are several lists of symptoms, which have a different number of symptoms, but I believe only ten symptoms to be the most relevant and related to depression only, not being moody or something. So here they go one by one
#1. Feeling helpless or hopeless.
#2. No interest in the daily routine
#3. Changes in appetite and weight, both ways
#4. Changes in sleep
#5. Strong irritability or anger
#6. Loss of energy
#7. Felling yourself worthless or guilty
#8. Engagement in escapism behavior (gambling, dangerous driving or extreme sports)
#9. Problems with concentration
#10. Ache and pain for no reason.