Psychology is a science that is studied by someone who is not a qualifed medical practitioner.

Psychiatry is a science that is studied by someone who is a fully qualified medical practitioner.

Basically the science is the same, the students and therefore the qualified practitioner is different.

To put it another way:
A psychologist is someone who has studied psychiatry and who isn’t a medical practitioner.
A psychiatrist is a medical practitioner also qualified in psychiatry.

I looked the definitions for Psychology (and psychiatry and psychotherapy) from (from Ask.com):

• The Science of mental life

• The thought processes characteristic of an individual or group: ethos, metality, mind, mindset, psyche. See thoughts.

• Thoughts
  1 the content of congnition, the main thing you are thinking about
  2 the process of thinking (especially thinking carefully): thinking always made him frown
  3 the organized beliefs of a period or group or individual: 19th century thought
  4 a personal belief or judgement that is not founded on proof or certainty: my opinion differs from yours.

• psychotherapy the branch of psychiatry concerned with psychological methods, psychotherapy – the treatment of mental or emotional problems by psychological means

Psychiatrists are generally thought to deal with seriously confused and mentally disturbed patients while psychologists are thought to deal with slightly muddled thinking that may or may not be causing illnesses.
However the science behind the mentally disturbed or muddled thinkers is the same and from this science of psychiatry – other forms of ‘training’ the mind have been developed – ie Neuro Linguistic Programming and Hypnotherapy to name but a few.

Christians don’t generally like psychiatry because for them God is their psychiatrist. The bible is full of the human nature traits and how to avoid the pitfalls.

However I feel that the way the mind thinks and adapts to situations and other people is the crux of life and grossly overlooked until illness or abnormal behaviour strikes. God was/is the first person to realise this and this is why His book is full of ways to cope, absolutely chocker block full. It should be every psychiatrist’s handbook!