Post-Traumatic Stress Disorder is a dissociative disorder characterized by "intrusive thoughts" (the person can't stop thinking about it), nightmares, flashbacks, and so on. It's usually precipitated by a horrible and life threatening experience -- something sudden, unexpected, and disasterous. PTSD is best known as the reaction that people have after they have fought in an exceptionally messy warzone, but it is also now recognized as something that happens to people who are at "ground zero" during acts of terrorism, as well as what happens to children who have witnessed something exceptionally threatening such as the violent death of a parent, a car accident, or their own abuse situations.
This is, of course, an oversimplification, but if you look carefully at multiples, you can often see the signs of PTSD, although not everyone with PTSD is multiple -- most multiples who have suffered through trauma show signs of PTSD (nightmares and sleep disturbances are common enough amongst multiples as are flashbacks -- called "abreactions" for multiples).
PTSD generally takes a long time to work out and recover from, partially dependent on being able to share experiences with others who understand, who were there, or who had similar experiences. Multiples generally find talking to other multiples to be very beneficial, as do PTSD veterans, etc. (That's one of the reasons that we came to our above-stated conclusion about why PTSD happens... -- The Crisses)