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.
Yeah it was really hard to function in school. Can depression lead to psychosis?
Some individuals occasionally will have false beliefs and think they see or hear things that other people cannot.