"Most people experience post traumatic stress, not everyone develops the disorder"- David Strother (LSCSW/LCSW).
The disorder part of PTSD is what this section addresses.
For a clinical definition of PTSD click here or the link below.
Here I am going to describe PTSD from my own experience and understanding living with it:
Think of the human body as a self healing survival based organism, and human beings are part of a highly adapative, invasive, and resilient social survival based species (the human species).
When we experience, "normal" events in life there is an infomation component and an emotional component involved. The mind makes sense of the situation and stores it for later in the comparative memory bank, and the day goes on.
When we experience, "traumatic" events in life there is an information component and an overwhelming emotional component. The survial brain registers the situation as a threat to life and pays especially close attention to particular detail of the event, to try and make sense of it, and more efficently survive similar events in the future.
With trauma the memory of the event become, "fragmented" into pieces of the event, instead of one fluid memory, and the fragmented pieces stick out glaringly, whereas the rest of the details of the event / story seem to fade into the background. If the memory of the traumatic event remains in fragmented form it becomes, unresolved trauma.
Any reminders of the event can prompt the survival brain to become hyperfocused on the remembered details about the survived event, but not the entire event start to finish to gain a sense of settlement in the mind.
The survival system can activate just remembering the event, from a visual reminder of the event, a song that was playing during the event, the smell of a particular odor (cologne / exhaust fumes / etc) that was smelled during the event, etc.
Think about how well our bodies protect us, if we got sick after eating a particular food (the taste of the same type of food can activate the survial system into rejecting it as possible poison), etc.
When recalling a traumatic event (either voluntarially or involuntarially) the survival system floods the body with the original emotions of the event, in order to prime us to survive the event again.
The surival brain works from a state of hypervigialnce order to gain a survival advantage using what is known as Symbolic Representation to play match-game with possible threats (this is where you hear the term "Trigger(s)" commonly used) and the survival brain pre-emptively activates the survival system with emotions on a better safe than sorry basis.
When these emotions flood, the experience is relieved from a dissocated sense of awareness, much like driving somewhere on autopilot preoccupied in thought, only to arrive there safely and realize we didn't remember the drive at all.
If you consider that human beings are adaptable enough to normalize any environment that does not physcially kill us.
With PTSD, the person's stress response system had been activated so intensely for a long enough period of time to create a short cut for the survival brain to use to reactivate the stress response system without having to check in with the pre-frontal cortex to validate the threat.
The survival brain essentally tells the prefrontal cotrex, "Been there, done that, this is a known threat, shut up", even if the threat is not actually a threat at all, but only a similar possible threat, and this is where the traumitized mind can run in loops.
This pattern can continue for a lifetime until corrected, however the beauty part is correcting these patterns is the fast part, which is where we get into the healing sections. Identifying the pattern is what most often takes so long.