Schizophrenia is a psychological disorder that has been fully fleshed out in DSM 5. There are three phases of schizophrenia: prodromal (beginning), active and residual. Those who are in the active stage appear to not be grounded in reality and appear psychotic. While the symptoms are very impairing, they can improve quite a lot if they are treated.
Schizophrenia has been misconceived as a split personality disorder or a multiple personality disorder; it is not. Neither are the majority of people having schizophrenia violent or dangerous.