What is the difference between psychosis and schizophrenia?

Psychosis. Schizophrenia is a psychotic disorder not attributed to by organic (medical) causes for the symptoms, or by substance use. Psychosis could be primary symptom or caused by any of the above attributes (organic psychosis) or mood disorders such as bipolar do, or personality do's such as borderline personality.
They're similar. But not the same. Psychosis is a symptom, like fever is a symptom. Many conditions can have psychosis as a symptom, scores if not a100 different ones. Schizophrenia is a brain disease which has the symptom of psychosis at its'core.
Symptom vs disorder. Psychosis is a symptom--poor reality testing, hallucinations, delusions, etc. One of the disorders for which it is a symptom is schizophrenia, but there are other possible causes of psychosis ranging from toxins, metabolic disturbances, severe depression and mania.