Books and articles on introversion have received a lot of attention lately, and I'm sorry to say, there's a lot of misinformation out there. I love that we're talking about it, but let's try not to oversimplify the matter, shall we?
Here's my take, and for your reference, I've been certified in MBTI and somewhat seriously learning about Personality Theory, Temperament / Essential Motivators, the Neuroscience of Type, and Interaction Styles for about 3 years. Not PhD expert level yet, but not quite laywoman either.
Introversion and Extraversion, as explained by Carl Gustav Jung, describe
a) how we gain energy, and
b) where our mental energy flows first.
They can NOT be inferred by how many people we hang out with, they do NOT predict how much we talk, and they CERTAINLY don't indicate how many books we've read.
Everybody DOES both, but PREFERS one over the other.
People with Introversion preferences gain energy from their inner world of thoughts, ideas, principles, values, experiences, memories, and visions. When presented with a new object or new thought or new person, they are more likely to take a mental step back to observe. They are still able and likely to be brilliant presenters or trainers when the subject or group is important to them.
People with Extraversion preferences gain energy from the outside world of things, impressions, connections, patterns, ideas, and themes. When presented with a new object or new thought or new person, they are more likely to take a (mental) step forward to interact and involve. They are still able and likely to enjoy down time on their own or with few close friends when given the chance.
Yes, I believe there's also a third group who is ambivalent.
Why am I saying there are no Introverts?
Because we're more than that.
Where we gain energy and how it flows is only one part of the theory. Jung also talked about how we process information and make decisions. He called those the perceiving and the judging functions, there's four of them, and they can all be expressed in both introverted and extraverted attitudes. This brings us to a total of eight so-called function-attitudes.
Since Jung's days we now have some more data on type dynamics (how those function-attitudes interact with one another) and type development (how they show up and when). Dr. John Beebe makes a compelling case mapping the eight function-attitudes on eight different archetypes (representations of persons everybody understands / universal images), and when he strapped EEG caps on his subjects making them go through hours of activities, Dr. Dario Nardi found that people of different personality types use their brain in fundamentally different ways - again, mappable onto the eight different functions.
Dr. Linda Berens first turned me on to paying attention to Type language, and how using nouns sounds and feels more finite than using verbs. Since we're talking about processes, an action word is a better description. "Introvert" sounds like that's it, done. "Introverting your Sensing function" allows for a lot more awareness of the fact that it's happening in any given moment, and that perhaps 10 minutes from now, you might be extraverting your Thinking function. Notice the difference? So, please:
Don't believe the hype and think you can dumb personality theory down to two boxes. There are no Introverts or Extraverts. There is balance between the two, and we can't do both at the same time. All of us use some of our functions in the introverted attitude at least some of the time. And all of us use some of our functions in the extraverted attitude at least some of the time.