D.I.D. is a Psychological disorder where the brain creates several distinct personalities outside of the "main" personality.
Think Dr. Jerkyll and Mr. Hyde.
The personalities may or may not be aware of the other personality, and sometimes there are triggers to commence the change. Most often D.I.D. happens to those who suffer trauma and cannot cope with it. The brain reacts and creates a separate personality so that the person doesn't "Break" so to speak.