PTSD

Post-Traumatic Stress Disorder is a potentially debilitating anxiety disorder triggered by exposure to a traumatic experience such as an interpersonal event like physical or sexual assault, exposure to disaster or accidents, combat or witnessing a traumatic event.                                           - found here

This is a good basic overview of PTSD from the U.S. National Library of Medicine and PubMed Health 

WebMD provides a somewhat more expansive overview

The Wikipedia page has quite a bit of good information, but it also has a LOT of detailed clinical information (such as neurological activity and medical applications) that may not be as useful.

It is evident that early diagnosis and treatment can frequently keep the disorder from developing into a trauma. 

From a 2010 PBS documentary entitled 'This Emotional Life'

Another clip: Bob talks about the effects of PTSD on his life:

Lisa French was on the bus bombed in London in July 2009:

News interview with PTSD veteran:

Here a soldier talks about PTSD and describes some of his own symptoms - his explanatory information comes from the same sources I've listed above.