Freud founded psychoanalysis, which is a theory that explains why humans behave the way that they do and is also used to treat mental health concerns. He believed that childhood events had a larger impact on our lives than previously thought and that these events shaped who we grew up to be as adults.

Freud devoted much of his work to uncovering these events and other contributing factors to our personalities. He introduced words like denial and libido as well as phrases like anal personality to our vocabulary in his exploration.

In addition, Freud birthed the theory that the mind is basically a complex energy system, the structural investigation of which is the proper province of psychology today. Freud also articulated the concepts of the unconscious, infantile sexuality, and repression, to name just a few.
Link: Sigmund Freud Quotes