According to the dictionary, psychiatry is the study and treatment of mental illness, emotional disturbance, and abnormal behaviour.
A psychiatrist is a medical doctor with special training in psychiatry and mental health who can also prescribe medication. Some psychiatrists provide therapy but many do not and tend to focus on diagnosis/evaluation and/or psychiatric medication monitoring.
Psychology is the scientific study of the mind and behaviour.
A registered psychologist is a person with a minimum of a graduate degree in psychology (although there are several registered psychologists who also have doctorate level degrees), who has completed a minimum of 1600 hours of supervised practice, and in Alberta, has passed the Examination of the Professional Practice of Psychology. Psychologists focus on different therapeutic approaches, usually from the approach of combining biological, psychological and social factors to treat mental illness and issues. They do not prescribe medication.