The Economist explique.

The election of Pope Francis on March 13th was surprising for several reasons. He is the first pope from South America, making him the first non-European since the 8th century. He is also the only pope to take the name Francis—evoking the humility of St Francis of Assisi, a 12th century Italian monk. Most surprising of all, he is the only member of the Society of Jesus, a religious order dating from the 16th century, to become a pope. But just who are the Jesuits, exactly?