Psychology refers to the study of human behavior and the human mind, or the mental attitude of a group. Your parents will find it hard to understand the psychology of the teenager.
Psychology is more than just an academic subject. The word refers to the complex mental workings and states a person, group or activity. Marketers use the psychology of consumer behavior to sell you things you don't need. Some self-help books teach you how use psychology - mental strategies and tricks - on someone in order to win arguments or get ahead.