Dissociative identity disorder
According to the DSM-IV, the symptoms of DID are "the presence of two or more distinct identities or personality states" (which are often called "alters"), as well as forgetting things that people normally don't forget.  Also, the symptoms cannot be caused by drugs or normal childhood play.  People can report having a lot of alters, or just a couple.  Most people diagnosed with DID have less than ten, though some have said they have several thousand. A person diagnosed with DID cannot be diagnosed with another dissociative disorder.  Because people with DID hear voices of different alters, it may be confused with schizophrenia.  However, the two problems are very different and doctors can usually tell if a patient has DID or schizophrenia by asking the right questions.  DID is always diagnosed by what patients tell their doctors and what the doctor thinks this means.  There are no objective tests using blood or machines that can tell if someone has DID.  Though people have tried diagnosing and studying DID using medical imaging or brain scanning.