Introversion and extroversion are psychological modes of adaptation. In the former, the movement of energy is toward the inner world. In the latter, interest is directed toward the outer world. In one case the subject (inner reality) and in the other the object (things and other people, outer reality) is of primary importance.
Introversion is normally characterized by a hesitant, reflective, retiring nature that keeps itself to itself, shrinks from objects [and] is always slightly on the defensive. Extroversion is normally characterized by an outgoing, candid, and accommodating nature that adapts easily to a given situation, quickly forms attachments, and, setting aside any possible misgivings, will often venture forth with careless confidence into unknown situations. In the extroverted attitude, external factors are the predominant motivating force for judgments, perceptions, feelings, affects and actions. This sharply contrasts with the psychological nature of introversion, where internal or subjective factors are the chief motivation.
Extroverts like to travel, meet new people, see new places. They are the typical adventurers, the life of the party, open and friendly. The introvert is essentially conservative, preferring the familiar surroundings of home, intimate times with a few close friends. To the extrovert, the introvert is a stick-in the mud, a spoil-sport, dull and predictable. Conversely, the introvert, who tends to be more self-sufficient than the extrovert, might describe the latter as flighty, a superficial gadabout.