An introvert is, to oversimplify, a person who gains his or her energy
from the outside world
and interpersonal interaction sap the strength from such a person, requiring him or her to retreat into isolation
in order to mentally and physically recover from the stress
and emotional overbearance.
The definition provided by Carl Jung, founder of modern type theory, among other claims to fame, is somewhat more inspiring (taken from unknown source):
"...Introversion for Jung is interest in the subject, while extraversion is interest in the object. This raises the important metaphysical question about the nature of subject and object. Although Jung would have found both question and answer in Schopenhauer, he was not interested in burdening his psychological analysis with particular metaphysical doctrines. "'Interest in the subject' thus simply means the internal states, whether of one's self or of others, are the primary way that the introvert relates to the world, while the extravert relates through objects. One consequence of this is that when introverts are interested in objects, this tends to isolate them rather than relate them to others -- objects for an introvert are private rather than public."
In other words, while the extravert
is interested in the object
itself, the introvert is generally more interested in the meaning
behind the object -- why
it is there, what
constitutes it, where
it came from, how
it got to be&where it is, etcetera.
The introvert generally prefers to exist in a mostly self-controlled environment, where the amount of sensory input can be heightened or dampened at will, as, with his/her deep and sensitive involvement with stimuli, the introvert is very succeptible to sensory overload. When in a situation overly-full of superficial, mind-clouding input, the introvert's sensors are completely overloaded and he or she must retreat as soon as possible to a state of solitude, in order to recover. The extravert, meanwhile, with his or her looser, surface-oriented sensors, needs all of the outside input he or she can get, to avoid an unhappy bout of sensory deprivation. As an introvert might define it, "to an extravert, it's not so much the quality of the information as the quantity." This is true in a sense, but an extravert would likely have a very different way of looking at it.
Anyway, the way it narrows down is that the introvert tends to have astonishing depth of focus and understanding, but a relatively narrow amount of experience. Being bombarded with sensory information confuses and rattles him. The extravert tends to have a very wide range of experiences and contacts but in trade for a lack of detailed comprehension. Sitting on one topic and getting personal with it makes him fidgety. For more info past this shallow overview, look to Jung or Myers-Briggs.