It has been said that extroverts gain energy from being around people and Introverts lose energy from being around people.
The idea of two different types of personalities was first described by Carl Jung and further researched by Hans Eysneck and are defined by base levels of arousal. Extroverts with a lower level of arousal need to do more exciting things that would mentally and physically exhaust an introvert. While an introvert might be content with a leisurely walk or a quiet read an extrovert might need to hit the gym.