Can you tell me the difference

Sorry to trouble u all, but, can someone please throw some light on the subject of the difference between a psychologist and a psychiatrist. What does the word "clinical" mean in front of it. I was summonds by my neuro- consultant. I have m.s and as I was practicing with the injector pen (that I have to try and help my m.s) he has now made me an urgent appointment and I don't want to, but what repercusions will follow if I don't go.The dictionary and peoples views differ greatly on what these two jobs are, and what they are meant to achieve.Thanks.K