I think what you describe as short term is also called situational depression, and yes, I think most people experience this. I think there are some other times when it's natural to get depressed, like during adolescence when hormones are fluctuating. What you describe as long term is Major Depressive Disorder, sometimes just called "clinical depression." That can fluctuate too, based on circumstances, but it's always there to a certain degree and can be quite severe for weeks, months, etc., hence manifesting as long term.