Jack and Annie are the main characters in the popular children's book series called Magic Tree House, written by Mary Pope Osborne. They are siblings who embark on exciting adventures through time and space using a magical treehouse filled with books. Each book takes them to different historical periods and locations, where they learn about various cultures, events, and important figures. Throughout their journeys, Jack, the older brother, often takes on the role of the cautious planner, while Annie, the younger sister, is more spontaneous and adventurous. Together, they solve problems, face challenges, and discover the power of knowledge and imagination.