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?
Major depressive disorder
Major depressive disorder (MDD), also known simply as depression, is a mental disorder characterized by at least two weeks of low mood that is present across most situations. It is often accompanied by low self-esteem, loss of interest in normally enjoyable activities, low energy, and pain without a clear cause. People may also occasionally have false beliefs or see or hear things that others cannot. Some people have periods of depression separated by years in which they are normal, while others nearly
ray detector technologies.
DQE especially affects one's ability to view small, low-contrast objects. In fact, in many imaging situations, it's more important to detecting small objects than is limiting spatial resolution (LSR) - the parameter traditionally used to determine how small an object one can visualize. Even if a digital system has very high LSR, it can't take advantage of the resolution if it has low DQE, which prevents the detection of very small objects.
A study comparing film/screen and