The Psychiatrist is a physician who has been trained and has the Psychiatry specialty title. He/She is responsible for the diagnosis and treatment of psychiatric disorders. He/She is therefore the person someone with mental health problems should address, in order to get a proper diagnosis and a proposal of the appropriate treatment.
Psychologist is the scientist dealing with human psychology as it is formed through the development of the individual via his/her family and his/her social environment.
Via the terminology, the Psychologist holds the theoretical background of the psychology of the individual and what is defined as normal, but does not have the capacity of the healing intervention when something is out of the normal.
In most cases however, psychotherapy works out effectively by offering stable solutions and relief to smaller or bigger psychiatric or psychological issues.