Psychology is the search for understanding, and there are many goals of psychology. These goals include describing, understanding, predicting, and influencing human and animal behavior and mental processes. It also studies behavior which can be overt, visible, or covert, private.
I'm glad you asked. There are so many!
Well, who are the people that developed this?
First, there is Wilhelm Wundt. He was known as the "Father of Psychology" and he believed that the mind and behavior can be studied scientifically. I also came up with the term introspection which means to look inside yourself.
Next is William James, and he was known as the "Father of American Psychology". His main focus was how the mind functioned and his key contributions were functionalism, which is how the mind works and adapts to new experiences, and Pragmatism, which is if an idea works, is it valid and useful.
The third major psychologist was Sigmund Freud, he is also the most well-known. He created the Ego, which respects authority but also gives into pressure, Super-ego, which wants to always do what is right, and ID, which does whatever without realizing consequences. He also created psychoanalysis which treats mentally ill people by analyzing their unconscious minds.