As the science of behavior and mental processes, psychology is an extremely broad discipline. It seeks to both describe and explain every aspect of human thought, feelings, perceptions, and actions.

There are many major subdivisions of psychology. Developmental psychology is concerned with processes of growth and change over the life course, from the prenatal period through old age and death. Physiological psychology focuses on the body's neural and chemical systems, studying how these affect thought and behavior. Experimental psychology investigates basic psychological processes, such as learning, memory, sensation, perception, cognition, motivation, and emotion. Personality psychology looks at differences among people in traits such as anxiety, aggressiveness, and self-esteem. Clinical and counseling psychology specializes in diagnosing and treating psychological disorders, whereas social psychology focuses on how people influence each other's thoughts and actions. Finally, industrial and organizational psychology studies problems in the workplace and other kinds of organizations.

A number of fundamental questions cut across the various subfields of psychology, unifying them with similar themes. Some fundamental questions are (1) Is behavior caused more by inner traits or by external situations? (2) How do genes and experiences interact to influence people? (3) How much do we stay the same as we develop and how much do we change? (4) In what ways do people differ in how they think and act? (5) What is the relationship between our internal experiences and our biological processes?

As a science, psychology relies on the scientific method to find answers to questions. This method involves careful observation and collection of data, efforts to explain observations by developing theories about relationships and causes, and the systematic testing of hypotheses (or predictions) to rule out theories that aren't valid.

Pseudosciences, such as astrology or palm-reading, do not base their practices and theories on evidence and research.