Psychology has a varied history and has its routes mainly in philosophy and biology. It has gone through trends of research and ways of thinking, e.g. psychoanalysis in the 1890s (developed by Freud), behaviourism in the early 20th century (a reaction to the subjectivity in psychoanalysis - behaviourism aimed to be scientific) and humanistic psychology in the 1950s (a reaction to the behaviourist movement largely - the idea was to look at people as whole beings
, rather than just studying small areas). There is a very informative article on psychology on Wikipedia, which is worth having a look at
for more information.