The terms introvert and extrovert (spelled extravert by Carl Jung) were originally employed by Sigmund Freud and given significant amplification later by Jung. The terms refer to "attitudes" and show how a person orients and receives their energy. In the extroverted attitude the energy flow is outward, and the preferred focus is on people and things, whereas in the introverted attitude the energy flow is inward, and the preferred focus is on thoughts and ideas.
Thus, one who is introverted is more likely to spend time alone or in contemplation, as these activities are rewarding. They may avoid social situations entirely, not because of shyness, but because they choose to.
While most people view being either introverted or extroverted as a question with only two answers, the reality is that this is probably more of a scale, with people falling on both ends and in between. The term ambivert was coined to denote people who fall more or less directly in the middle and exhibit tendencies of both groups. An ambivert is normally comfortable with groups and enjoys social interaction, but also relishes time alone and away from the crowd. Ambiverts tend to be moderate thinkers and weigh more than one side to an issue. Most have warm but controlled personalities.