Susan Cain - The Power of Introverts

"Introverted leaders often deliver better outcomes than extroverts do."

These are Carl Jung's terms, and are to be viewed as a single continuum.  As one is increased, the other decreases.  Let's sum them up:

Extroversion is "the act, state, or habit of being predominantly concerned with and obtaining gratification from what is outside the self".

Introversion is "the state of or tendency toward being wholly or predominantly concerned with and interested in one's own mental life".