Sadness occurs when one is affected by unhappiness or grief, expressing sorrow. Sadness is a feeling. It is an expressible, physical thing that occurs as a response to some stimulus.
Depression, however, is very different. For me, depression has no feeling. When I feel depressed, I feel empty inside. I don't feel passionate about anything. It's as though the world is turning around me and I'm the sole spectator. It's very difficult to physically express depression. Yes, there can be crying or pain. Occasionally, we might find we have our arms wrapped around ourselves, trying to hold the pieces together.
Sadness is part of a process. Grieving can be healthy, a release. Depression is not a part of a normal mental process. When grief or pain gets out of control, it can lead to depression. But that is not a step in mental healing.
Sadness usually involves a timeline. Something happens to cause sadness. You feel sad. You experience some sort of release. You are happy again. You can even perform in a normal way when you're sad. Depression can strike without warning, and it has no final step for closure. You feel down mentally, emotionally and physically - sometimes to the point where it's difficult to perform everyday actions.
You've hear of "good days and bad days." This is nothing but the truth. Some people may feel sluggish and empty one day, and they will be active and accepting the next day. But "good days" don't mean the depression is gone. That's why it's called a mental illness. If a cancer patient feels healthy one day, it doesn't mean the cancer disappeared.
The most helpful thing families and friends can do is stay educated. Try to be empathetic: walk a mile in a depressed person's shoes. By increasing understanding and willingness to learn, we may see the stigma dissipate. And who knows? Perhaps it could lead to better treatment for those with depression.

