Dissociative Identity Disorder (DID), formerly known as Multiple Personality Disorder, is a complex psychological condition that is characterised by the presence of two or more distinct personality states or identities within a single individual. These distinct identities may have their own names, ages, histories, and characteristics, including noticeable differences in voice, posture, and even gender. DID is often a result of severe trauma during early childhood, usually extreme, repetitive physical, sexual, or emotional abuse.

One of the key features of DID is dissociation, a mental process which produces a lack of connection in a person's thoughts, memory, and sense of identity. Dissociation is often thought of as a coping mechanism, where the person literally dissociates himself or herself from a situation or experience that's too violent, traumatic, or painful to assimilate with their conscious self.

The symptoms of DID vary and can include:

1. The presence of two or more distinct identities or personality states. Each may have its own personal history, self-image, behaviors, and preferences.
   
2. Amnesia or memory gaps, where the person cannot recall significant personal information or events, which is too extensive to be explained by ordinary forgetfulness.
   
3. Depersonalization: Feeling detached from one’s thoughts, feelings, and body.
   
4. Derealization: Experiencing the external world as unreal or distorted.
   
5. Identity confusion or identity alteration: A sense of confusion about who one is.

It is important to note that DID is a controversial diagnosis and not all mental health professionals agree on its characteristics or origins. Some experts believe that DID is a complex form of post-traumatic stress disorder (PTSD).

The precise cause of DID is not known, but it is typically linked to extreme stress or trauma experienced during critical stages of childhood development. DID is believed to be a coping mechanism—the individual dissociates themselves from a situation or experience that's too violent, traumatic, or painful to integrate with their conscious self.

Diagnosis and treatment of DID can be challenging. Treatment generally involves psychotherapy with the aim of integrating the different identities into one primary identity. Therapy also focuses on coping with trauma and improving relationships. The use of medication may be used to treat concurrent symptoms or disorders, such as depression or anxiety.

Understanding and compassion are crucial when interacting with someone who has DID. It's a complex disorder that requires a nuanced approach, both in clinical settings and in broader societal contexts.