From Abnormal psychology (http://www.purgatory.net/merits/index.htm)
These include four recognized varieties: psychogenic amnesia, psychogenic fugue, multiple personality, and depersonalization disorder. Again, these are highly publicized in the media but they are relatively rare.
Amnesia is the temporary or permanent loss of a part or whole of memory. When this is due to extreme psychosocial stress, it is labeled psychogenic amnesia. This stress is most often associated with catastrophic events.
There are four sub-categories of psychogenic amnesia: localized amnesia, selective amnesia, generalized amnesia and continuous amnesia.
This is most often an outcome of a particular event. The disease renders the afflicted unable to recall the details of an usually traumatic event such as a violent incestual rape. This is undoubtably the most common type of amnesia.
As it's name implies, this is similar to localized amnesia except that the memory retained is very selective. Often a person can remember certain general occurences of the traumatic situation, but not the specific parts which make it so.
These less common forms of amnesia are defined as when the diseased either forgets the details of an entire lifetime, or as in the case of continuous amnesia, they can't recall the details prior to a certain point in time, including the present.
Recognized as an independent clinical syndrome, a fugue is simply the addition to generalized amnesia of a flight from family, problem, or location. In highly uncommon cases, the person may create an entirely new life.
Defined as the occurence of two or more personalities within the same individual, each of which during sometime in the person's life is able to take control. This is not often a mentally healthy thing when the personalities vy for control.
Symptoms are of course somewhat self-explanatory, but it is important to note that often the personalities are very different in nature, often representing extremes of what is contained in a normal person. Sometimes, the disease is assymetrical, which means that what one personality knows, the others inherently know.
This is the continued presence of feelings that the person is not oneself or that they can't control their own actions. While these are common human feelings, it is labeled a disorder when it is recurrent and impairs social and occupational function.
Symptoms are a change in the person's perception of themselves. The disease may incur strange feelings that one's limbs are not shaped or sized correctly. It also may cause a sense of being outside of one's body. While self-awareness is extremely distorted, "reality-testing functions" remain intact which denotes an absence of delusions or hallucinations. The person perceives others as mechanical as if they existed in a dream. The afflicted have a constant worry about going insane.