Trauma is something that leaves a seemingly-indelible mark on you. It can affect your feelings, your thoughts, your sleep, and your relationships. It may even affect how you think of yourself.
Typical examples of trauma include rape and war atrocities. These are but the tip of the iceberg. Bad events from childhood, death of a loved one, threats, bullying, witnessing a crime, domestic violence, specific encounters with scary things (perhaps a big dog), being in a dangerous situation, isolation, sudden break-ups, and medical issues like a surgery or amputation can all be traumatic. The abrupt nature of the event is traumatizing because the victim has no warning, no time to prepare defenses or use coping skills. This leaves a mark on the mind that causes a facet of the mind to attempt to prepare defenses after the fact. When those don't work, the mind becomes confused and attempts to replay the event until the defenses work; they cannot work, as the mind cannot create safety where it was not, and the result is flashbacks (when waking) and nightmares (when asleep).
With trauma, the mind's gears grind to a halt and attempt to free themselves by repeatedly forcing the same action. Unlike other mental health issues, trauma is specialized in that the therapist must adjust that facet of the mind that is malfunctioning at the precise point of stuck-ness. This is not done by reliving the experience - or the mind would have done this during a flashback or nightmare. The commonly accepted technique is narrative therapy, which is the retelling and reframing of events so that the victim becomes the hero. This is a bending of the truth - or creating a better truth. A more effective method is Rapid Resolution Treatment. With RRT, you will be talking, laughing, losing stock in Kleenex, and staying present, in the moment, while telling - but not living - the experience, with the kinds of emotions you would have in an everyday conversation.
Institute for Rapid Trauma Resolution.