In his 1920s work Psychological Types, Swiss psychiatrist Carl Jung popularized the terms "introverted" and "extroverted" describing personality types. Introverts recharge by spending time alone, and they lose energy if they're around people for long periods. Extroverts, on the other hand, gain strength from social interactions and actually exhaust themselves if they're alone. Both of these types have their good and their less perfect qualities, so there's no point in arguing which side would win in the Introvert VS. Extrovert battle.