Introversion and extroversion (or extraversion) are terms first coined by psychiatrist Carl Gustav Jung in Switzerland of the 1920s.
According to Jung, an extrovert seeks intensive contact with the outside world. An introvert instead turns the psychic energy inwards. Jung believed that no one is 100% extravert or 100% introvert instead we carry both traits.
However, most minds tend to lean either on one side or the other. If both characteristics are equally present, we can speak of someone who has an ambivert personality.