Introverts and Extroverts

I had a really interesting lunchtime conversation on Friday. Part of the conversation was about introverts and extroverts. I'd considered myself an introvert throughout high school and college, so I was somewhat surprised that one of the people at the table said I was definitely an extrovert. The more I think about this, the more I wonder how can one really tell? With some descriptions of the differences between the two, I still find myself more aligned with the introversion side of things.

There was talk about confort zones, about not being embarassed, about personal growth, about fear. But I'm not really sure how all that relates to this introvert/extrovert thing. Are you an introvert if you prefer to read? Are you an extrovert if you prefer to play team sports? How much introspection must you do before you're considered an introvert? Which are you if you prefer to communicate via computer games? It's all very confusing. I haven't done much looking, but I still haven't read a description of the differences between the two that I find compelling.