Depression is a term to describe a wide range of mood disorders that create psychological distress in the sufferer.
It can ebb and flow with the severity of the symptoms and can often be erratic - sometimes mild, sometimes bad, sometimes very bad, sometimes bearable. Symptoms recede and return. This can last a few weeks, or months or even years.
Depression is, to feeling low; as having the flu, is to the common cold! If you have ever had true flu, then you know what I am talking about.