Elizabeth Bernstein in the Wall Street Journal: 

Ambiverts have introverted and extroverted traits, but neither trait is dominant. As a result, they have more balanced, or nuanced, personalities. They aren’t the folks yammering your ear off. Nor are they the totally silent ones happily ensconced in the corner.

Ambiverts move between being social or being solitary, speaking up or listening carefully with greater ease than either extroverts or introverts. “It is like they’re bilingual,” says Daniel Pink, a business book author and host of Crowd Control, a TV series on human behavior, who has studied ambiversion. “They have a wider range of skills and can connect with a wider range of people in the same way someone who speaks English and Spanish can.”