So is it like a hallucination where you really feel yourself back to the moment of trauma, or you just feel crap because something reminded you?

I mean, the trauma for me ended over 20 years ago, but it went on for most of my childhood and basically most of my childhood and teenage memories refer to it in some way.  I am surrounded by memories.  So what is a flashback and what is just having intrusive memories or emotions?

Does anyone have links or a good definition of what an actually flashback means?  Or thoughts from personal experience?

Is this used as a gauge of how affected you were, the fact that a person doesn't get flashbacks means well it couldn't have been that  bad?  Or, must you have flashbacks in order to have PTSD?