Jellies who know: How is Depression (the mental health condition) different from feeling depressed?

Someone close to me is experiencing a change in mental health and has been diagnosed with depression. For this person it manifests as being anti-social and unmotivated to get things done, but also some occasional fatalistic/dark thoughts (e.g. “It won’t matter if I don’t get this done..I won’t be alive by the time it’s due anyways..”)

As someone who hasn’t experienced depression but occasionally has felt depressed (..and to some small degree, haven’t we all? Winter is dark, dreary and/or after sad life events feeling depressed is normal, right?) can you please help me try to understand how it feels differently to have clinical depression? Can you find words to describe how it feels to you?
Aside from medication, what has helped ease/resolve/prevent your depression most?