DID stands for Dissociative Identity Disorder...here in the United States DID is
(found at http://behavenet.com/dissociative-identity-disorder )

Diagnostic criteria for 300.14 Dissociative Identity Disorder

A. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).

B. At least two of these identities or personality states recurrently take control of the person's behavior.

C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.

D. 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). Note: In children, the symptoms are not attributable to imaginary playmates or other fantasy play.

this standard of diagnostics and definition of DID will be changing with the new DSM that comes out around may 2013 here in the USA.

Treatment for DID is the same as any other mental disorder....

therapy
medication for problems that sometimes accompany DID like PTSD symptoms of depression, anxiety, insomnia and sometimes psychosis, those meds are called antidepressants, antianxiety, sleeping pills and anti psychotics.
integration (some people have the ability to choose whether to integrate or not and for others it just happens as a normal part of the therapy process)

there is no medication to treat DID directly but that doesnt mean you just have to live with it. people do heal from having it, I used to have DID, now I am one whole person again thanks to years of therapy, meds and learning how to not use dissociation to abnormal levels, meeting and solving my problems face on.