Depression is when a person experiences low mood and an aversion to activities that affects their thoughts or behaviors. It can be very limiting. 
I have experienced it in the past. It's very hard to live with. How many people in the US suffer from it?
I'm not sure how many people suffer but I know it is a major crisis. A depressed mood can be a symptom of a disorder such as major depressive disorder or dysthymia. 
What are some other symptoms? I suffered from it for years. I was put on Zoloft.
Symptoms can include sadness, anxiety, a feeling of emptiness, worthlessness and a sense of hopelessness.  
I had a medium level case of it. What are some of the treatments used?
I know that most people are treated with anti-depressants and therapy. major depressive disorder can have a negative effect in a person's work, school, and their health.