PTSD, namely Post Traumatic Stress Disorder, is an anxiety disorder that can develop after exposure to one or more terrifying events that threatened or caused grave physical harm.
This may involve a persons death, a threat to someone's life, serious physical injury, or threat to physical integrity.
In some cases it can be caused by extreme psychological and emotional trauma, aside from any physical harm, but most of the time the two are combined.
Post traumatic stress disorder is different from traumatic stress because it has less intensity and does not last as long.
Post Traumatic Stress Disorder has been recognized in the past as railway spine, shell shock, traumatic war neurosis, and post traumatic stress syndrome.
To learn more about it in today's society, I suggest you use some of the online resources available. One of my personal favourites can be found at
VideoJug.com, where you can find in interview with an expert in the subject discussing
Post Traumatic Stress Disorder.