PTSD

Post-traumatic stress disorder (PTSD) is an anxiety disorder that can result from exposure to a traumatic event. In her book, Trauma and Recovery, Judith Herman describes traumatic events as "extraordinary, not because they occur rarely, but rather because they overwhelm the ordinary human adaptations to life. Unlike commonplace misfortunes, traumatic events generally involve threats to life or bodily integrity, or a close personal encounter with violence and death. They confront human beings with the extremities of helplessness and terror, and evoke the responses of catastrophe."

According to the DSM-IV, there are three different symptom clusters: intrusive recollection, avoidance/numbing, and hyper-arousal.

Intrusive recollection can involve intrusive thoughts, feeling as though the trauma were happening again (flashbacks), recurrent dreams and distress when exposed to reminders of the event.

Avoidance can include avoiding thoughts, people, or places that are associated with the traumatic event. Numbing may involve an inability to recall parts of the trauma, feeling detached from one's self, others or the world.

Hyper-arousal may include insomnia, irritability, concentration difficulties, hyper-awareness of surroundings or an exaggerated startle response.

In order to be diagnosed, symptoms from each of these categories must be present for more than a month and cause significant disturbance in functioning. PTSD can occur immediately following exposure to trauma or there may be a delay in onset.

This site lists all of the DSM-IV-TR criteria for PTSD and can probably answer any questions you might have about them. Additionally, the APA is currently drafting the fifth edition of the DSM and is considering changes to the PTSD criteria. You can see the proposed revisions here.