Basically, an introvert is a person who is reserved, quiet, and solitary. Introversion and its opposite, extroversion, also spelled extraversion, form the ends of a continuum that describes one aspect of every person's personality. The concept was popularized by the work of psychologist Carl Jung.
Jung described an introvert as a person whose psychic energy is directed inward. While most modern day psychologists do not believe in the existence of "psychic energy" per se, they agree that a person with this type of personality is more concerned with and interested in his or her own thoughts than in the external world. Introverts often come off as shy and prefer to spend time on their own or with one or two close friends. They typically feel more energy and can work more productively when alone.
While it is generally considered more desirable to be an extrovert in Western society, and extroverted traits are encouraged, being an introvert has significant benefits. These people often excel in higher education, in which the ability to spend large amounts of time with one's own thoughts is a great asset. They are also often capable of forming very deep and close friendships.
Psychologist Hans Eysenck postulated that the introvert experiences more brain activity than the extrovert and is therefore constantly at a higher state of stimulation without turning to external sources. Therefore, the large crowds of people that the extrovert is bored without can be overstimulating to the introvert or difficult for him or her to pay attention to. One study showed that these people experience greater blood flow in areas of the brain controlling logic and problem-solving.
There are some drawbacks to this personality type as well. Such people often find it difficult to make friends with others and may become lonely. They may find events with large groups of people uncomfortable and may be perceived as rude or stand-offish. Introverts will probably not excel in careers in which engaged personal interaction with strangers is essential. A study by psychologist David Myers found that these people are less likely to be "happy" than extroverts, although the reason for the correlation is unclear.
People who are curious about where they fall on the personality spectrum can find many self-tests available in books and online. They should keep in mind, however, that most people don't fall on the extremes of the spectrum and actually have features of each type.