I've known three people with Dissociative Identity Disorder (formerly known as Multiple Personality Disorder). One of these persons gave a different name for her condition (I do not remember what she said it was) and said that it differentiated from typical Multiple Personalities Disorder by saying that her personalities were not as strongly formed as full personalities, never had been and that she had always been aware of all of them...unlike many people who are unaware of the other personalities and seem to "lose time" when another personality is in control. So, according to this one individual there are other types, but I have not heard of any in the DSM (the psychiatry manual used to classify mental illnesses).
Usually multiple personality disorder stems from extremely traumatic events. Although after this has become a "normal" coping mechanism for a person to handle the trauma in their lives it could become easier for personalities or "splinters" to form from less traumatic, but still stressful events.
There are also mental illnesses such as schizophrenia or bi-polar (manic-depressive) that some may confuse with split personality, but are not. A schizophrenic may hear voices, for example, but they are not voices of other personalities within them. Someone with bi-polar may have such wide mood swings that can give the impression of an almost different "person" but this is a change in mood, and not an alter-personality with his or her own memories, tastes, etc...even different allergies than the other personalities.
There are also several other dissociative disorders, such as Dissociative Amnesia, Dissociative Fugue, etc. But these do not involve having separate personalities within one individual.