Multiple personality. MPD, now called dissociative identity disorder is characterized by two or more distinct identity states known as alters in addition to the patient him/her self. These identity states will seem fragmentary and partially unaware of the others, each with varying degrees of disturbance, The alters tend to be more active and aggressive, while the patient's core personality will be more passive.
D.I.D. Dissociative identity disorder is a severe form of dissociation which produces a lack of connection in a person's thoughts, memories, feelings, actions, or sense of self. It is thought to stem from trauma experienced by the person with the disorder.
Now Dissociative Dis. Once called multiple personality disorder, it's rare and controversial (forensic issues). Criteria require being recurrently controlled by 2+ discrete identities or personality states, accompanied by memory lapses for important information. It can not be caused by alcohol, drugs, etc. They may or may not know of each other, so you may never know. Others close to you might know. It is rare!
DID. Dissociative identity disorder (aka multiple personality) is often the result of severe trauma. One aspect of the personality will evolve to deal with some things, and other "personalities" with other aspects of life. Clinical hypnosis by a clinical psychologist can be helpful with this uncommon condition.