You've always known at least one person back in high school who would sit at the back of the room and say almost nothing for the entire school year. Or probably you have been that one guy or girl who sits at the back of the classroom and never says anything for the entire school year. Whatever your stance is, Introverts exist.
They enjoy the company of their thoughts and prefer to listen rather than speak, they tend to be shy and have only a few close friends. What if I told you introversion is developed over time, it does not burst out of the ground fully grown and it is nurtured until it becomes what it is most people are familiar with.