Extrovert and introvert are probably the two terms from the MBTI (Meyers-Brigg Type Indicator) that you are most familiar with. These two words are often used, but commonly misunderstood. That's why Pinkie Pie and Fluttershy will be helpful in explaining them.
Like most extroverts, Pinkie Pie gets charged up from being around people and doing lots of things. She feeds off the energy around her, and in return charges up her companions. She sometimes charges into situations without thinking them through completely or completely planning out all her projects before she starts. When she sees someone hurting, she has an instinctive need to cheer them up, even if it makes her look silly, and she sees any good news as a reason to celebrate.
As an ENFP, Pinkie Pie is a classic extrovert. She lives above the bakery where she works in the center of Ponyville. She lives to throw parties and is the premiere event planner in town. She can usually be seen exuberantly bouncing along as she does her business, and amusing her neighbors in the process. (Some would also describe her as having a severe case of ADHD.) Pinkie Pie has never met a stranger and loves to be around ponies of all kinds.
At the other end of the spectrum, we have Fluttershy, the ISFJ. She lives on the edge of Ponyville near the Everfree Forest where she spends her days as an animal caretaker. If anyone in Ponyville needs help with a pet, they call on Fluttershy.
While she enjoys attending Pinkie Pie's parties, Fluttershy prefers quiet, one-on-one conversation or time alone with her animals. As an introvert she spends energy whenever she is around large groups of people, such as at Pinkie Pie's parties, and recharges by escaping into her own little world. Where Pinkie Pie charges into any situation, Fluttershy is more likely to take the time to consider her options before committing to a course of action. Though, she is a little awkward when she first meets people, once she is comfortable, Fluttershy is a great confidant.
Pinkie Pie's strength lays in her ability to do what is right and what needs to be done without thought to what people will think of her, or what it may cost her. Fluttershy is the friend who will always be there when you need someone to talk to. The kids that our volunteers work with need someone who will speak up for them, even if it is about something unpleasant. But they also need someone who is willing just to listen to them. It doesn't matter if you are a Pinkie Pie or a Fluttershy, the kids need you.
by Melody K. Coehoorn