Dissociative Identity Disorder
Dissociative Identity Disorder (DID) is a "disturbance in normal integrative functions of memory, identity, and consciousness",  and it is the most complex of the Dissociative Disorders. This mental disorder is included in the Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5) , and the ICD-10 .
Information sources - folklore as opposed to expert literature
The non-professional world obtains information from popular media, which is too often inaccurate, and from professional literature which is more likely to be accurate, but may be incomplete. What is missing from some expert sources, such as the DSM, is the description of etiology and functional dynamics.
The term Multiple Personality Disorder is confusing
Normal behavioral and personality development
Dissociative Identity Disorder (Multiple Personality Disorder) is not multiple personalities, it is one personality, but often when people think of this disorder, they misunderstand it to be a proliferation of selves. No one can have multiple personalities, but everyone does have multiple personality states that make up their one personality, [4 p.7] even those people with Dissociative Identity Disorder.
"In infancy, behavior is organized as a set of discrete behavioral states, such as states of sleep and waking, eating, elimination, and so on. If an infant and toddler are appropriately cared for, then these "behavioral states become linked over time and grouped together in sequences," [4 p.8] until a unitary personality sense emerges that can fluently shift from one task-focused mental state to another, dependent upon need. [4 p.59] As a child's brain develops, various personality states come to share a sense of having a common identity, while retaining the ability to easily move from one personality state to another. [4 p.89] As Ross and Ness concluded in a landmark 2010 study, "symptom patterns in Dissociative Identity Disorder are typical of the normal human response to severe, chronic childhood trauma, and have ecological validity for the human race in general." 
Abnormal personality development
Dissociative Identity Disorder is usually a posttraumatic developmental disorder caused by prolonged stress during early childhood, prior to the normal integration of behavioral states.  Coping by dissociation is an innate ability all children have, but if the nurturing and compassion a child needs in response to early childhood stress is missing, [4 p85-108] they will frequently dissociate stressful memories from consciousness. Trauma memory can be experienced later as separate dissociated personality states. The earlier the abuse begins the more likely one is to develop Dissociative Identity Disorder.
The vast majority of Dissociative Identity Disorder cases are due to some form of physical childhood abuse.  Studies show it is possible for severe neglect and emotional abuse during the early years of childhood, that lacking physical or sexual use, to also cause this mental disorder. In addition, rare cases of unintentional trauma during this important period in life are thought to have led to the development of Dissociative Identity Disorder. Examples would include: an accident, hospitalization or even the death of someone essential to the child. 
Alter personalities (alters)
An important way that alters and normal personality states differ, is that alters are compartmentalized states. An alter has "a sense of its own identity and ideation, and capacity for initiating though process and actions." [4 p.57] Dissociated states are found in all the dissociative disorders, but states that are so dissociated as to have the characteristics of an alter are unique to Dissociative Identity Disorder. A variety of distinguishing physiological measures have been found in alters  including: "visual acuity, medication response, allergies, plasma glucose levels in diabetic subjects, heart rate, blood pressure, galvanic skin response, muscle tension, immune function, electroencephalography, evoked potential patterns, functional magnetic resonance imaging activation, brain activation (and) regional blood flow."  Alters also often have "different psychophysiological organizations, such as different allergies, taste preferences, handedness, eyesight and prescriptions for glasses, and responses to medications." [4 p.57]
Common types of alters include: hosts (always more than one in DID), child parts, inner-self helpers, introjects, protectors, managers, suicidal states, twins, otherkin, dead alters, persecutors, gatekeepers and care-takers. Each part in the system has a job that they perform - a needed job rather than a preferred job. [4 p.55-66] Systems can range in number from a few to thousands. It is generally considered that the more horrendous the abuse was, the more alters will be created to aid in survival of the child.
Symptoms, signs and diagnosis
The newest diagnostic criteria are those found in the DSM-5.  They include the following, plus specifiers for "prominent non-epileptic seizures  (PNES or pseudoseizures)  and/or other sensory-motor (functional neurologic) symptoms. This edition of the DSM-5 emphasizes fragmentation of identity, memory, and consciousness and at the same time add the "disruptive effect of symptoms on consciousness and a broader definition of symptoms." 
1• Disruption of identity characterized by two or more distinct personality states (one of course can be the host, since this is also a dissociated state) or an experience of possession, as evidenced by discontinuities in sense of self, cognition, behavior, affect, perceptions, and/or memories. This disruption may be observed by others, or reported by the patient.
2• Inability to recall important personal information, for everyday events or traumatic events, that is inconsistent with ordinary forgetfulness.
3• Causes clinically significant distress and impairment in social, occupational, or other important areas of functioning.
4• The disturbance is not a normal part of a broadly accepted cultural, religious practice, or part of the normal fantasy play of children.
5• The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during alcohol intoxication) or a general medical condition (e.g., complex partial seizures). 
The DSM description of Dissociative Identity Disorder may well be inadequate. An in-depth study, by Paul Dell reported non-observable symptoms in those with Dissociative Identity Disorder including: state dependent amnesia, conversion symptoms, self-alteration, derealization, depersonalization, flashbacks, trances, identity confusion, awareness of alters, voices, thought withdrawal and insertion, made impulses, feelings and actions and non psychotic auditory and visual hallucinations. The study reports that the description in the DSM is "deficient because it omits most of the dissociative phenomena of Dissociative Identity Disorder and focuses solely on alter personalities." 
More information - Dissociative Identity Disorder on the Trauma & Dissociation wiki