Extraversion and introversion are often presented as being binary personality states: either you're an extrovert, or an introvert. But the reality is that people's social styles exist on a spectrum, with those easily exhausted by dealing with other people firmly on the introversion end and the social butterflies on the extraversion end. But a lot of people fall somewhere in the middle. Sometimes they find social interactions energizing, sometimes they find them tiring; it all depends on the situation.
These people in the middle of the spectrum are the ambiverts.
Ambiverts can enjoy going out with their friends to parties or big concerts, sometimes, but they also sometimes like spending a night at home alone with their favorite video game or a book and glass of wine to recharge. They aren't especially quiet, but they aren't loud, either. They instinctively stand up for themselves, but they're not domineering. They can hold up their end of a conversation, talking and listening in equal measure.
It's possible that some people who suffer from social anxiety disorder and who have identified themselves as introverts will, as they manage their anxiety with medication or therapy, come to realize they're actually ambiverts.
Though ambiverts don't dominate social situations like the extroverts, it turns out that according to research by Adam Grant, ambiverts actually make better salespeople than extroverts do. Grant believes that this is because ambiverts focus better than extraverts, listen better to what their customers actually want, and have a softer touch. They don't come off as too pushy or overbearing, particularly when they're dealing with introverted customers. Such a social depth perception advantage could easily extend to other endeavors that are commonly thought to be ruled by extroverts.