I think of introversion and extroversion as the sources of energy for a person. The energy preference typically ends up having an effect on the person's habits in general.
An introvert recovers from stress, exhaustion and life's worries by being by themselves and thinking about it.
An extrovert recovers through interaction with other people. That sort of thing revitalizes them.
Some introverts are fully able to enjoy the company of other people, but it wears them down eventually. Likewise, extroverts can enjoy being alone, but eventually they will need to revive themselves with interaction.
Of course, there is a spectrum for both. Some people are extremely introverted, while others are so mildly introverted that they can pass for an extrovert.
also, thanks for the add, Moonchild =)