Introverted and Extroverted is a distinction on where people draw their energy. It is not about (although often mistakenly thought to be) shyness, confidence, or self-esteem.
Introverts draw their energy from within (hence, "intro"). They are comfortable being alone and comfortable using their minds as exercise and entertainment. Studying and thinking is natural to them, and doing so gives an introvert energy and fuel. On the other hand, interacting with other people spends an introvert's energy. Introverts can literally feel exhausted by having to be at a party or having to do an activity that requires them to project and reach out to others.
This does NOT mean that they don't like these activities! Introverts can enjoy being social just like extroverts, the distinction is all about the energy. Introverts can enjoy being at a party, but may feel tired afterwards, or they may prefer to stay home because they know they will GAIN energy by being home, but LOSE energy by being out.
Extroverts are the opposite: they draw their energy from outside themselves (hence, "extro"). They GAIN energy from being around other people, and lose it by being alone or by being forced to study alone. This doesn't mean extroverts aren't smart, either.
Introversion and Extroversion is a sliding scale, not two separate camps. It is an inherent nature, you cannot "pick" which one you are. About two-thirds of the population is believed to be extroverted, and one third introverted. There are advantages and disadvantages to both. If you want to learn more, you might want to read "The Introvert Advantage: How to thrive in an extrovert world" by M. O. Laney. It has an excellent explanation and discussion on who introverts and extroverts can understand each other and work together.