What is the main difference between sadness and depression?

Sadness vs depressio: Sadness is a symptom of the syndrome of depression, but can be just an isolated fact in a period of life if not accompanied by, among others, insomnia or sleeping life away, excessive fatigue, loss of appetite or excessive appetite, loss of pleasure in doing things you used to like, feeling bad about self, feeling guilty....If sadness lasts too long and too severe, it will lead to depression.

Severity: Most people use the term depression to refer to a more severe case of sadness. Depression typically means the depressed mood and loss of interest or pleasure are causing clinically significant distress or impairment in social, occupational or other importnat areas of functioning.