What is the difference between "feeling down" compared to actual depression?

I honestly can't tell the difference because they both feel the same. I just want to know what signs to look out for and be aware what's going on because I have had this overall sense of heartache and disappointment for the past several days and I feel like it has been consuming my mind, any answers would be greatly appreciated.
What is the difference between "feeling down" compared to actual depression?
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