This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
Multiple personalities [also known as multiplicity, MP (for Multiple
Personalities) and MPD (for Multiple Personality Disorder, something
most people who ARE multiple find insulting)] means basically
what it says: someone who has multiple persons/personalities living
inside of one body. These are referred to here as alters. (As of the
DSM-IV, this condition is called Dissociative Identity Disorder, but most
people here will probably not use the official term.)
Multiples, and people who come to deal with them, are aware of these
different alters as completely separate people, rather than
different facets of the same person. For instance, someone who was
utterly professional and cold at work but who was a rocking metal-head
partier at home would not (necessarily) be multiple just because
of the wide difference in the way they acted in different situations.
[They might, however, be considered dissociative.] In fact, some
multiple systems have within them alters who are very similar to one
another, differing so slightly that sometimes the multiples themselves
have difficulty telling them apart.
The differentiating factors can vary greatly. One of the possible ways to
tell them apart is names, but many multiples have several alters of the
same name. Some might _all_ have the same name.
The various alters can be of the same or different gender as the
body, including being of no gender at all. They often have different
likes, dislikes, tastes, etc. They often have different body language, speech
patterns, and sometimes, voices. There are cases where eye color differs.
Many have different abilities, physical and mental. They may be
of widely varying ages. Sometimes they will self-identify as having
separate racial/cultural backgrounds or native languages. Often they have
different religions. Some studies have shown that different alters have
different brain wave patterns (EEG readings), although this is a topic
There can be as much variance within a multiple system as there would
be in your average room full of people.