Functionalism in psychology is also sometimes called Functional Psychology. It deals with how well individuals adapt to their surroundings. It is different than other branches of psychology as it offers an uncontrolled view. It is often used to develop and express theories that are not testable because there are to many variables which are out of our control.
Functionalism came about because there was a strong emphasis on the testable physiological and anatomical elements of psychology. Functionalism offered an alternative approach.