Introversion is often mistaken for shyness. A Venn Diagram of the two might show a large intersection, but they are different qualities of personality. A shy person is generally timid, quiet and retiring. Introverts are not necessarily any of these things.
Introverts typically are misunderstood. Extroverts, who outnumber introverts by about 3:1, consider introversion to be a pitiable condition which must be remedied. A recent study apparently showed that the "problem" with introverts is that they are unable to take pleasure in things which are presumed to be unquestionably pleasant. Milling about in large crowds of noisy people, attempting to draw the attention of as many of them as possible to oneself, is presupposed to be one of those pleasant activities.
It is not the case that introverts avoid that which is pleasant and embrace that which is unpleasant. They simply do not find typical extroverted activites pleasant. An introvert would rather be known by one person than be merely noticed by fifty.
Introverts maintain small circles of close friends. They do not feel that they are denying themselves friendship. Quite the contrary, an introvert with a large circle of friends would constantly feel as if he were neglecting most of them, resulting in feelings of guilt. Despite the concerns of well-meaning extroverts, being a social butterfly, even a successful one, is very stressful for an introvert. Certainly there are people who avoid social activities for fear of rejection, but it would be incorrect to identify this group as "introverts."