this is a question in which there are many different answers, a simple answer for this is 'Psychology is the science of human thought and behaviour', whereas a more in depth answer is 'the scientific study of experience and behaviour. psychology draws together systematic studies of experience and behaviour using a wide range of methods, and focusing on many different angles and levels of experience.'
it seems to me that the answer to this question changes with with every book, i dont think i have seen to the same, which can be very confusing<>/FONT>