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A very basic definition is that introverts have a preference for quiet, minimally stimulating environments.
This means that introverts tend to enjoy quiet concentration, listen more than talk and think before they speak. They tend think more and focus on quality over quantity in most things and tend to focus intently on a single project at a time (i.e. friendships and how they engage projects and hobbies at work/during free time).
Extroverts are energized by social situations and tend to be multi-taskers who think out loud and therefore need others' feedback to validate their ideas or nudge them in the right direction.
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I highly recommend the book, especially if you think you're an introvert or have difficulty accepting yourself as one. It really helped me see that it was OK being introverted and that there were certain advantages that extroverts don't have