Hi, depression is a state of low mood and I experience it some days. Have you ever felt it?
Yes I have, I had post partum depression. Have you?
Yes, sometimes. Its a temporary reaction to life events that can be distressing. 
Yes and other times it can be very long term. Medication helps alot if you are on the correct one. 
Sadly, its also a side effect of some drugs and medical treatments. so it can be long term indeed.