According to Wikipedia, introversion and extraversion are measures of personality traits. There are two different interpretations of the concept of extraversion/introversion:

According to Hans Eysenck, introverts are withdrawn, self-absorbed, and uncommunicative people. This interpretation is commonly used in psychology as well as in MBTI. (so the types in MBTI are somewhat different from personality types in socionics)
According to Jung, introversion and extraversion are psychological modes of adaptation. The extroverted way consists of a high rate of reproduction, with relatively low defensiveness, which constantly encourages the extrovert to waste away and embed himself in everything around him;
The introvert's opposite method of adaptation is to defend oneself from any external influences, to refrain from any expenditure of energy, but to create for oneself the most advantageous position in the world. The Jungian treatment of introversion and extraversion is also used in socionics.