Multiple Personality Disorder is a SEVERE mental illness. It results from a person being severely abused. Their normal personality "fragments". For instance, on the one hand they're supposed to LOVE their parent (or whoever is abusing them). On the other hand, they can't escape the violence, and showing anger will just bring MORE violence. Their mind winds up separating various parts of a healthy integrated personality into separate "personalities" that often aren't aware of each other. So, for instance, the anger part becomes a separate "person" that is not aware of the "love" person, etc. Usually VARIOUS personalities are aware of SOME of the other personalities. There normally is one personality that is the "main" personality and tends to be the one that most commonly manifests. One personality might go and buy something that others aren't even aware of.
There is always one personality that IS aware of ALL the other personalities, but that isn't the "main" personality. Normally, curing MPD takes YEARS of INTENSE therapy. In most cases MPD isn't even diagnosed, people just think "John's a nut case" and try to avoid him.
The most famous case known to the public was Sybil. A movie The Three Faces of Eve was based on her. However, she actually had 16 different personalities. They made it "3" for the movie because they thought if they said 16 no one would take it seriously.